Add gluster network disk snapshot cases

Replace domain disk with gluster network disk, then do snapshot, including:
 - external disk snapshot to local file
 - external disk snapshot to glusterfs target file
 - system checkpoint (external snapshot with both mem and disk) snapshot
 - multiple disk snapshots with one as gluster network disk
 - multiple gluster disks snapshot to gluster target disks
 - iscsi network disk snapshot
 - snapshot with 'snapshot' attribute set in domain disk xml