watch: Also look for xz compressed tarballs

debian/watch

 version=3
 
 opts="uversionmangle=s/\.-rc/~rc/;s/\.+$//,pgpsigurlmangle=s/$/.asc/" \
-    http://libvirt.org/sources/libvirt-([\d\.]+)(-rc\d)?\.tar\.gz
+    http://libvirt.org/sources/libvirt-([\d\.]+)(-rc\d)?\.tar\.[gx]z