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>
core: Extract Validator creation and apply usages

There are several places creating a MultipleStorageDomainsValidator
based on a VM and DisksImage List in the same manner.
Some new code for bellow bzs does the same, as will other patches. This
patch removed future code duplicity by extracting the
duplicated code into VmCommand.

Change-Id: I3a6d04fe1a670c814b44bad5304a894faa8f8510
Relatee-To: https://bugzilla.redhat.com/1054175
Signed-off-by: Vered Volansky <vvolansk@redhat.com>