migration: Inhibit virtio-balloon for the duration of background snapshot

The same thing as for incoming postcopy - we cannot deal with concurrent
RAM discards when using background snapshot feature in outgoing migration.

Fixes: 8518278a (migration: implementation
  of background snapshot thread)
Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Reported-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210401092226.102804-3-andrey.gruzdev@virtuozzo.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>