Why doesn’t libvirt “Usermode Networking” start on boot

Running Ubuntu 16.04, the default virbr0 interface is not started by default for me on boot, meaning I have to start it manually using the command,

$ sudo virsh net-start default

Not a big deal perhaps, and I'm aware of ways to workaround this, but the output of the following command suggests to me that virbr0 is configured to start automatically:

$ virsh  net-list --all
 Name                 State      Autostart     Persistent
----------------------------------------------------------
 default              active     yes           yes

Does anyone have any pointers where to look next to help debug this?

Update:

Some selected lines seen in /var/log/syslog following boot that don't help me, but may mean something to someone:

systemd-udevd[368]: Could not generate persistent MAC address for virbr0: No such file or directory
NetworkManager[862]: <info>  [1508409794.6955] device added (path: /sys/devices/virtual/net/virbr0, iface: virbr0): no ifupdown configuration found.
NetworkManager[862]: <info>  [1508409794.6956] device added (path: /sys/devices/virtual/net/virbr0-nic, iface: virbr0-nic): no ifupdown configuration found.
NetworkManager[862]: <info>  [1508409794.7711] device (virbr0-nic): state change: unmanaged -> unavailable (reason 'connection-assumed') [10 20 41]
NetworkManager[862]: <info>  [1508409794.7725] device (virbr0-nic): state change: unavailable -> disconnected (reason 'none') [20 30 0]
NetworkManager[862]: <info>  [1508409796.0723] device (virbr0): state change: unmanaged -> unavailable (reason 'connection-assumed') [10 20 41] 
avahi-daemon[847]: Registering new address record for 192.168.122.1 on virbr0.IPv4.
NetworkManager[862]: <info>  [1508409796.0734] device (virbr0): state change: unavailable -> disconnected (reason 'connection-assumed') [20 30 41]
NetworkManager[862]: <info>  [1508409796.0739] device (virbr0): Activation: starting connection 'virbr0' (6a02deba-6a45-44f3-8c7d-92bb6f8c287b)
. . .                                                                                       
NetworkManager[862]: <info>  [1508409796.0829] device (virbr0): Activation: successful, device activated.
NetworkManager[862]: <info>  [1508409796.0833] device (virbr0-nic): state change: secondaries -> activated (reason 'none') [90 100 0]
nm-dispatcher: req:5 'up' [virbr0]: new request (2 scripts)                        
nm-dispatcher: req:5 'up' [virbr0]: start running ordered scripts...               
NetworkManager[862]: <info>  [1508409796.0877] device (virbr0-nic): Activation: successful, device activated.
nm-dispatcher: req:6 'up' [virbr0-nic]: new request (2 scripts)                    
nm-dispatcher: req:6 'up' [virbr0-nic]: start running ordered scripts...           
avahi-daemon[847]: Interface virbr0.IPv4 no longer relevant for mDNS.              
avahi-daemon[847]: Leaving mDNS multicast group on interface virbr0.IPv4 with address 192.168.122.1.
kernel: [   11.827450] virbr0: port 1(virbr0-nic) entered disabled state           
NetworkManager[862]: <info>  [1508409796.4151] device (virbr0): state change: activated -> unmanaged (reason 'connection-assumed') [100 10 41]
avahi-daemon[847]: Withdrawing address record for 192.168.122.1 on virbr0.         
nm-dispatcher: req:7 'down' [virbr0]: new request (2 scripts)                      
nm-dispatcher: req:7 'down' [virbr0]: start running ordered scripts...             
kernel: [   13.245387] device virbr0-nic left promiscuous mode                     
kernel: [   13.245390] virbr0: port 1(virbr0-nic) entered disabled state           
NetworkManager[862]: <info>  [1508409797.8565] device (virbr0-nic): state change: activated -> unmanaged (reason 'unmanaged') [100 10 3]
NetworkManager[862]: <info>  [1508409797.8565] device (virbr0): bridge port virbr0-nic was detached
NetworkManager[862]: <info>  [1508409797.8566] device (virbr0-nic): released from master device virbr0
NetworkManager[862]: <info>  [1508409797.8620] devices removed (path: /sys/devices/virtual/net/virbr0-nic, iface: virbr0-nic)
nm-dispatcher: req:8 'down' [virbr0-nic]: new request (2 scripts)                  
nm-dispatcher: req:8 'down' [virbr0-nic]: start running ordered scripts...         
NetworkManager[862]: <info>  [1508409797.8990] devices removed (path: /sys/devices/virtual/net/virbr0, iface: virbr0)

Looks a bit like virbr0 is enabled and then disabled again during boot.