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>
I'm afraid there is a bug here to send None for timeout.
https://github.com/autotest/virt-test/blob/master/virttest/aexpect.py#L1185
In this function, timeout=None may cause TypeError because time.time() is float.
Do I miss something?