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>
Maybe it is better to use the main_monitor way to get this monitor too? use the [-1] monitor in env_process seems not safe.