core: Storage validation when running stateless VM

When a VM is run as stateless, temporary volumes are created for each disk leaf.
Up till now there were no storage validations in this case. These were
added in this patch, along with some new functionality to the
MultipleStorageDomainsValidator, as well as related tests.

Change-Id: Id04203119cce475afeef7658db50e132b0c7036a
Bug-Url: https://bugzilla.redhat.com/1054175
Signed-off-by: Vered Volansky <vvolansk@redhat.com>