blockpull: Update create snapshot use xml file

Update make_disk_snapshot use snapshot-create with xml, this will
enable create snapshot file with gluster target.

Add bandwidth test cases and snapshot in mirror negative case.

Signed-off-by: Wayne Sun <gsun@redhat.com>
blockpull: Restart libvirtd after cleanup glusterfs

After several test runs, gluster cases could got fail for start vm
with error:

  error : virStorageFileBackendGlusterInit:611 : out of memory

and libvirtd process got memory leak, this will block the following
gluster testing cases, restart libvirtd in test case to help avoid
this.

This is a known issue:
https://bugzilla.redhat.com/show_bug.cgi?id=1143800

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