fix virsh.snapshot_create to return command().

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
fix virsh_snapshot for new virsh.snapshot_create

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
add test for virsh snapshot disk.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>