upstart: Don't forward requests for LXC_DOMAIN

Without this change, a request to *.LXC_DOMAIN that doesn't get a local
result from dnsmasq will be forwarded to its upstream server with the
potential of a loop.

Thanks to Ed for the patch on Launchpad (LP: #1246094).

Reported-by: Ed Swierk
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>