xm: Don't die when trying to conect the console to short-lived domains

xm: Don't die when trying to conect the console to short-lived domains

As observed by Mick Joran, if short-lived domain exits cleanly
then os.waitpid() will throw the following exception. This appears
to be because the child process that is used to start the domain
has detached from its parent.

OSError: [Errno 10] No child processes

Cc: Mick Jordan <Mick.Jordan@sun.com>
Signed-off-by: Simon Horman <horms@verge.ent.au>