core: Threshold validation for stateless VM

When a VM is run as stateless, temporary volumes are created for
each disk leaf. Threshold validation is added in this patch.

Change-Id: Ic15815abfe25bb5d78b5115e2d05bf040fdcfcdb
Bug-Url: https://bugzilla.redhat.com/1054175
Signed-off-by: Vered Volansky <vvolansk@redhat.com>