Improve filtering of Xen domain0 in libvirt-guests

The list_guests function in libvirt-guests uses 'grep -v' to filter
Xen domain0 from a list of guests. If domain0 is the only item in
the list, 'grep -v' returns 1, causing the 'stop' operation to fail
when action is 'suspend'. Improve the filtering by using sed to remove
domain0 from the list of guests.

tools/libvirt-guests.sh.in

@@ -121,7 +121,7 @@ list_guests() {
         return 1
     fi
 
-    echo "$list" | grep -v 00000000-0000-0000-0000-000000000000
+    echo "$list" | sed "/00000000-0000-0000-0000-000000000000/d"
 }
 
 # guest_name URI UUID