core: RemoveVM - clear snapshot_id from detached disks

Clearing snapshot_id from detached disk images
when removing a VM.

Change-Id: I03fe42fd16067ceda87027b96948e746d0045229
RelatedTo-Bug-Url: https://bugzilla.redhat.com/1015170
Signed-off-by: Daniel Erez <derez@redhat.com>