[libvirt] [PATCH]: Update ruby-libvirt migrate binding to use rb_scan_args

It's really not a good idea to hand parse variable number of args to a ruby binding, like I implemented for the "migrate" method. Convert this to use rb_scan_args, which is the proper way to do it, and matches all of the other variable argument implementations in the binding. Signed-off-by: Chris Lalancette <clalance@redhat.com>