build: don't require avahi during install

build: don't require avahi during install

See: https://bugzilla.redhat.com/show_bug.cgi?id=785269

The specfile requires avahi during install if libvirt was built with
avahi support, but there are many situations where it is undesirable
to install avahi due to security concerns. This patch requires only
the avahi-libs package, which is needed by libvirt to call the
function that tries to attach to the avahi daemon, but will instead
silently fail because the avahi-daemon is in the main avahi package,
and that package isn't installed.