Make avahi startup more robust.

Make avahi startup more robust.

If the hostname of the current virtualization machine
could not be resolved, then libvirtd would fail to
start.  However, for disconnected operation (on a laptop,
for instance) the hostname may very legitimately not
be resolvable.  This patch makes it so that if we can't
resolve the hostname, avahi doesn't fail, it just uses
a less useful MDNS string.

Signed-off-by: Chris Lalancette <clalance@redhat.com>