Ovirt self-hosted engine High Availability

The vCenter VM won't be migrated to the remaining host, it will be restarted on the remaining host, so no vMotion will occur. It's also dependent on the VM Restart Priority. If the VM Restart Priority for the vCenter VM is disabled then it won't be restarted on the remaining host. If the VM Restart Priority is set to anything but disabled then the vCenter VM will/should be restarted on the remaining host. Note that the VM Restart Priority is dependent upon the available resources on the remaining host and the Admission Control setting (with or without DRS) so you want to set the vCenter VM restart priority to high to ensure that it is started on the remaining host.

Also note that a vMotion doesn't occur for the VM's on a failed host. VMware HA restarts the VM's on the remining host, it does not migrate them (with vMotion) to the remaining host.

Have a read here for more information:

http://pubs.vmware.com/vsphere-4-esx-vcenter/index.jsp?topic=/com.vmware.vsphere.availability.doc_41/c_useha_works.html

If you can afford downtime, why not simply backup the VM, install oVirt and restore the VM into an oVirt based VM?

Also, if the "small" box is going to be the engine, you can simply install the engine on it while the original VM runs in vbox, attach the "big" box as a host, and then move the VM into oVirt.

Another option is the self hosted engine, basically, deploy engine as a VM on the "big" box, move the VM, and later maybe also add the "small" box as a second host, so you can have a two node cluster.