Remove backwards compatability logic to simplify the bindings

Remove backwards compatability logic to simplify the bindings

Rather than compile against old versions of libvirt and resort
to throwing 'not supported' exceptions at runtime, we instead
require all symbols and definitions to be available at compile-time
i.e. we require a much more recent libvirt.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>