qemu: migration: Don't access disk members without lock

qemu: migration: Don't access disk members without lock

The initiation of a synchronous block job in the NBD storage migration
code was placed after entering the monitor thus after the lock on the VM
object was unlocked. Thankfully nothing bad could happen in this
situation since the migration job prevents any disk detaches or other
modifications of the domain object.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>