Simplify connection management
Simplify connection management

Generate the boilerplate for wrappign a connection in an object with
macros. Use a generic 'conn_attr' method to get at the connection of all
objects carrying one.

pronix/ruby-libvirt,pronix/ruby-libvirt,klynch/ruby-libvirt,klynch/ruby-libvirt,libvirt/ruby-libvirt,libvirt/ruby-libvirt,libvirt/ruby-libvirt,pronix/ruby-libvirt,klynch/ruby-libvirt