spec: require dbus-daemon when using libvirtd in Fedora

spec: require dbus-daemon when using libvirtd in Fedora

https://bugzilla.redhat.com/show_bug.cgi?id=830201

In older Fedora, the spec file for libivrt depended on avahi, which
included avahi-daemon, which in turn depended on dbus.  But now that
avahi libs and avahi-daemon are (correctly) in separate pacakges,
and since we REALLY don't want a mandatory dependency on avahi-daemon,
and considering that our init scripts require the messagebus service
from dbus, we need to explicitly require dbus ourselves.

* libvirt.spec.in (Requires): Add dbus for libvirt-daemon.