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>
No, so many virsh commands have different running times, we can't judge that, so it's better to use None. The proposal of this timeout is for these potential dangerous commands, such as pmsuspend, migration, etc. Let the tester to set this value is better.