[libvirt] [PATCH] Don't generate python bindings multiple times w/ parallel build

The attached patch was posted by Michael Marineau at: https://bugzilla.redhat.com/show_bug.cgi?id=472702 It prevents generating the python bindings multiple times in a parallel build. Works fine in my testing, so I figured I'd pass it along here for better exposure, since it has been languishing in bugzilla. Thanks, Cole