[libvirt] [PATCH]: Implement ruby-libvirt storage pool discovery binding

Attached is a pretty simple patch to implement the virConnectFindStoragePoolSources() binding for ruby-libvirt.This capability went into libvirt-0.4.5, so any version of libvirt that has storage API support should have support for this call. There's not much more to say; it works in my testing, and the implementation is pretty straightforward. Signed-off-by: Chris Lalancette <clalance@redhat.com>