Require avahi as an rpm dependancy

As seen in https://bugzilla.redhat.com/show_bug.cgi?id=746111

If compiled with the daemon and avahi support, then it's better
to require avahi at the rpm level to avoid daemon startup errors.

libvirt.spec.in

@@ -272,6 +272,9 @@ Requires: %{name}-client = %{version}-%{release}
 Requires: module-init-tools
 # for /sbin/ip & /sbin/tc
 Requires: iproute
+%if %{with_avahi}
+Requires: avahi
+%endif
 %endif
 %if %{with_network}
 Requires: dnsmasq >= 2.41