core: Added VM memory validation on snapshot

Up till now there were no storage validations when creating a snapshot
of a running VM with the VM's memory. These validations are now added to
the relevant command and validators.
Relevant tests were amended/added accordingly.

Change-Id: I8fc127147bdea8737fecb034c88079521b8a8bd6
Bug-Url: https://bugzilla.redhat.com/1119022
Signed-off-by: Vered Volansky <vvolansk@redhat.com>