virt.qemu_monitor: Add an API for terminating monitor connection

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.env_process: Close monitor socket after test

The socket connection is stll alive after a test finishes,
normally it works well.
But there is one situation that the next test case tries to
re-connect to these monitor sockets after unpickled from env
file, even though the former monitor socket connection is
still established. In this case auotest will report a error
of "Could not find (qemu) prompt", though the monitor works
well in fact.

To avoid this problem, this patch just close all monitors'
socket connections at the end of a test case, and let the
following test case try to re-connect if needed.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>