A VM operation (or destroy and clone, which hold the whole compute) waits
for its compute's lock while a migration holds it. When the migration
moved the VM's record to the destination, the waiting operation went on
there holding only the source compute's lock, unordered with a
reconciliation, migration or disk attach on the destination. After taking
the locks it now checks the VM is still on the locked compute and
otherwise answers busy, so the retry locks the right one. Clone also
checks again under the lock whether the user may clone the VM, and
checks the quota and the clone's settings against the VM as it is then:
its template mark, size or the user's rights may have changed meanwhile.
