CVE-2013-6456: Avoid unsafe use of /proc/$PID/root in LXC shutdown/reboot code

CVE-2013-6456: Avoid unsafe use of /proc/$PID/root in LXC shutdown/reboot code

Use helper virProcessRunInMountNamespace in lxcDomainShutdownFlags and
lxcDomainReboot.  Otherwise, a malicious guest could use symlinks
to force the host to manipulate the wrong file in the host's namespace.

Idea by Dan Berrange, based on an initial report by Reco
<recoverym4n@gmail.com> at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=732394

Signed-off-by: Eric Blake <eblake@redhat.com>