Finish the cleanup of libvirt.c; all uses of virLib*Error have
now been converted to more canonical conventions.

* src/libvirt.c: Use virReportError in remaining errors.
(virLibConnError, virLibDomainError): Delete unused macros.
* cfg.mk (msg_gen_function): Drop unused names.
Signed-off-by: NEric Blake <eblake@redhat.com>