[libvirt] [PATCH]: ruby-libvirt migration fixes

Attached is a relatively simple patch to the ruby-libvirt bindings with some bugfixes for the migrate call. The first problem was that there was no way to pass a "nil" through to the underlying virDomainMigrate() call. This is important because generally the "dname" and "uri" parameters end up being NULL. This patch also makes all the parameters beyond the first 2 optional. I've tested this out by live migrating a KVM guest between two hosts, and this now does what I expect. Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt;