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.

daemon/libvirt-guests.init.in

@@ -25,6 +25,7 @@
 
 sysconfdir=@sysconfdir@
 localstatedir=@localstatedir@
+libvirtd=@sbindir@/libvirtd
 
 # Source function library.
 . "$sysconfdir"/rc.d/init.d/functions
@@ -232,6 +233,12 @@ stop() {
     : >"$LISTFILE"
     for uri in $URIS; do
         echo -n $"Running guests on $uri URI: "
+
+        if [ "x$uri" = xdefault ] && [ ! -x "$libvirtd" ]; then
+            echo $"libvirtd not installed; skipping this URI."
+            continue
+        fi
+
         list=$(list_guests $uri)
         if [ $? -eq 0 ]; then
             empty=true