virt: Add catch_monitor for ScreenDump and VmRegister threads.

We have many thread need send monitor command. So Add a
catch_monitor for ScreenDump and VmRegister threads to reduce
acquire exclusive lock fail.

Signed-off-by: Feng Yang <fyang@redhat.com>
As catch_monitor is always add as last monitor in code.
+        if params.get("catch_monitor", "no") == "yes":
+            if "catch_monitor" not in params.get("monitors"):
+                params["monitors"] += " catch_monitor"

So use [-1] should be save.

We also can update the vm.monitor() function to let it support return monitor by name.