virsh: command timeout support

Set timeout in testcase for certain virsh commands(virsh.dompmsuspend() may hang there if guest/host not support pm-suspend well, etc.)

Signed-off-by: Yanbing Du <ydu@redhat.com>
Yes, you are right.
'None' will overwrite the default value '60' when calling ```cmd_status_output```, thanks, will update it.