[libvirt] [PATCH]: fix ruby-libvirt bindings when virConnectNum* returns 0

Attached is a simple patch to fix a problem I ran into when using the ruby-libvirt bindings with libvirt 0.4.3. Basically, if you call any of the virConnectList* functions with a "max" of 0, it returns "Invalid Arg". To get around this, modify the ruby-libvirt bindings to return an empty list if we get num == 0 when calling the corresponding virConnectNumOf* function. This should solve: https://bugzilla.redhat.com/show_bug.cgi?id=451666 Signed-off-by: Chris Lalancette <clalance@redhat.com>