qemu: Properly set //cpu/@migratable default value for running domains

qemu: Properly set //cpu/@migratable default value for running domains

Since active domains which do not have the attribute already set were
not started by libvirt that probed for CPU migratable property, we need
to check this property on reconnect and update the domain definition
accordingly.

https://bugzilla.redhat.com/show_bug.cgi?id=1857967

Reported-by: Mark Mielke <mark.mielke@gmail.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>