virttest: Add a thread for record vm register info in env_process

Add a thread as screendump to record vm register info get for vm
monitor. This debug log will be helpful when the guest is hang.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
shared.cfg: Update store vm register related parameters

Set the default parameters in base.cfg. Please update it if you
don't need this in your test.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>