libvirt-guests: Don't throw errors if libvirtd is not installed

libvirt-guests: Don't throw errors if libvirtd is not installed

When only client parts of libvirt are installed (i.e., no libvirtd
daemon), libvirt-guests init script in its default configuration would
throw seriously looking errors during host shutdown:

Running guests on default URI: error: unable to connect to
'/var/run/libvirt/libvirt-sock', libvirtd may need to be started: No
such file or directory
error: failed to connect to the hypervisor

This patch changes the script to print rather harmless message in that
situation:

Running guests on default URI: libvirtd not installed; skipping this
URI.