Systemd-Networkd – How to Configure Interface Aliases

Oh dear... I think you're trying to use the Xen bridging stuff, which is just dire. Set network-script network-dummy in xend-config.sxp, then go with this config:

iface lo
iface lo inet loopback

# Local network, cable labeled M3
auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.1.184
    netmask 255.255.255.0
    gateway 192.168.1.1

# cable labeled M1
iface eth1 inet manual
    hwaddress ether 00:19:5B:33:86:D5

# cable labeled M2
iface eth2 inet manual
    hwaddress ether 00:19:5B:33:86:D3

auto br-eth1
iface br-eth1 inet manual
  bridge_ports eth1

auto br-eth2
iface br-eth2 inet manual
  bridge_ports eth2

What you then do is tell the domUs to use either the br-eth1 or br-eth2 bridge (as appropriate). Given that you've got cable labelling happening, I'd change the bridge names to something more useful, like perhaps m1 and m2.

You have multiple interfaces with addresses on the same subnet with similar netmasks. The odds are a different route is being used for different packets, causing confusion. If you look at the output of

ip route show

You've probably got multiple default gateways as a result. For each set of IPs on the same subnet, give one of them the netmask you have, and the rest a netmask of 255.255.255.255. Those other interfaces will then only be used for packets originating or destined for that address and not others on the subnet.