virttest.qemu_vm: Catch all exceptions when sending quit to monitor in destroy

When host or guest under heavy load, the kill process will very slow.
After send the kill cmd inside guest. VM process will quit during following
steps in function destroy and will raise very strange error message like:
 error: (9, 'Bad file descriptor')
when trying to send the quit command to monitor.

So catch all kinds of exceptions and judge if the process is alive or not.
Log warning message in the log file instead of through errors.

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