Support "openstack server migrate [--live <host>] <instance>

The OpenStack nova instance model supports the concept of migration and "live migration" where an instance can be moved either out of a host, or to another host.

The original 'migrate' command simply evacuates an instance from a compute node, and nova chooses where it ends up based on the configuration/options in nova. The instance is stopped, moved, and then started (booted) on the new host.

The live migration allows the host to be chosen, and attempts to do it 'live' (i.e a state-full pause, transfer, and then resume on the new host). The implication for LXD is that the hosts have concrete CRIU implementations so that the lxc container can have a state-full snapshot taken, moved, and then restarted on the new host.