core: avoid pk violation during import vm

When importing a vm (not as a new entity) while having a disk with the
same id, pk violation occurs - this patch adds a validation to prevent
it from happening.

Bug-Url: https://bugzilla.redhat.com/902040
related to Bug-Url: https://bugzilla.redhat.com/890922
Change-Id: I84afa8167357b96ce47bc1340a64561a6d232b2d
Signed-off-by: Liron Aravot <laravot@redhat.com>