core: create OVF store as non shareable on gluster domain

oVirt doesn't support the creation of shareable disks on gluster
storage domains. Therefore the OVF store disks on gluster domains should
be created as non shareable.

Change-Id: I8e5d05a80061b963d61e09919824830b3624d3e6
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1134366
Signed-off-by: Liron Aravot <laravot@redhat.com>