core: validate disks existence on create snapshot

CreateAllSnapshotsFromVmCommand:
Added validation for specified disks existence.
* Included disks IDs in CanDo message.
* Updated tests accordingly.

Change-Id: I4d75ad139d048f83b08d9289e43d909b29f89695
Bug-Url: https://bugzilla.redhat.com/1057143
Signed-off-by: Daniel Erez <derez@redhat.com>