spec: If installing default network, restart libvirtd

spec: If installing default network, restart libvirtd

If libvirt-daemon-config-network is installed while libvirtd is already
running, the daemon doesn't notice the network. Users then have to
manually restart libvirtd (or reboot) to pick up the network.

Instead let's trigger a daemon restart when the package is first installed.
Then the default network is available immediately if libvirtd was already
running.

https://bugzilla.redhat.com/show_bug.cgi?id=867546