core: use GuidUtils for spliting memory volumes representation

Memory volumes, either for hibernation or for snapshots with memory,
are represented as comma-separated String of Guids. This patch changes
the way this kind of representation is split to Guids such that it would
use GuidUtils#getGuidListFromString at VmCommand#removeMemoryVolumes.

Change-Id: Id8a12ec75d09ce2369a5ccc68b384ce69a45d849
Signed-off-by: Arik Hadas <ahadas@redhat.com>