snapshot_disk: Add glusterfs network snapshot cases

Replace default disk with glusterfs network disk, then do snapshot
with xml file, use snapshot_xml module to handle the snapshot xml.

Add test cases for new option '--disk-only' for snapshot with
xml file. When with '--disk-only', the <memory/> element should not
be specified in the xml file.

Add test cases with two gluster disks and do external disk snapshot
to gluster target file.

Add netfs pool with gluster backend test cases.

Move external snapshot cases with <memory/> element in the xml to
sys_checkpoint filter, which indicate snapshot with both mem and
disks.

When attach raw disk and use xml to do snapshot, update driver type
to qcow2 as it's the only support external snapshot format.

When domain disk is 'block' type, update disk type to 'file' and
source file to external snapshot file in snapshot xml.

As external disk snapshot revert not supported for now, only do
internal revert for now and move the skip external disk snapshot
cases to pass.

As for iscsi pool on rhel6, destroy iscsi pool might fail with
can't find iscsi session for the pool, under such circumstance,
restart libvirtd to make sure the pool remove success, or
following case with create the same name pool will fail.

As gluster cases could also be affected by libgfapi bug, restart
libvirtd in cleanup to avoid the problem. More info:
https://bugzilla.redhat.com/show_bug.cgi?id=1143800

Signed-off-by: Wayne Sun <gsun@redhat.com>
snapshot_create_as: Add gluster and iscsi network disk test cases

Replace domain disk with gluster network disk, then do external disk
snapshot to local file.

By default, memory external snapshot without diskspec option is
external system checkpoint snapshot (both external memory and disk
snapshot).

Update filter 'live_memspec' with specific diskspec, which will
get the external disk snapshot file checked and cleaned.

Update filter 'no_metadata_with_memspec' with 'snapshot=no' in
diskspec, which only do memory external snapshot.

Remove guest agent channel if not required, this is for the negative
test case quiesce_without_unix_channel as by default now the vm is
with unix channel set, so remove it and got the case work.

Add iscsi network disk snapshot cases in cfg file, snapshot with lun
device current not support yet, so put it under negative case for
now.

Add test cases for disk with snapshot attribute. In domain disk xml,
the snapshot attribute accept 'no', 'internal' and 'external' value,
which used to allow or deny doing certain type of snapshot on the disk.
Also extend test cases with the attribute in file disk and gluster type
disk cases.

Signed-off-by: Wayne Sun <gsun@redhat.com>