core: ExportVmTemplateCommand storage allocations

MoveOrCopyTemplateCommand.canDoAction() small fix:
Fixed storage domain space verification. Using StorageDomainValidator
for some other storage verification, instead of (p)re-inventing the
wheel.
ExportVmTemplate inherits and therefore is affected by this.

Change-Id: Ic5de81b318d5b8a2e0da918b4a44ae2003ca4b06
Bug-Url: https://bugzilla.redhat.com/1136717
Signed-off-by: Vered Volansky <vvolansk@redhat.com>