core: Use correct validation for SD selection

When creating a snapshot, use new storage allocation validations for
the vm volumes when selecting a domain. Some code has been removed as a
result from this change.

Change-Id: Iac901a638358b9b08bcc6992d783a42cbfcd9bfa
Bug-Url: https://bugzilla.redhat.com/1119022
Signed-off-by: Vered Volansky <vvolansk@redhat.com>