[Libvir] [PATCH] #1: Add error handling and do quick wins (qemu, test)

This patch: (1) Adds the VIR_ERR_NO_DOMAIN and VIR_ERR_NO_NETWORK errors (the latter will be used later to make the same change to virNetworkLookup* functions). (2) Changes the documentation of virDomainLookup* functions to match the new behaviour. (3) Changes qemu & test drivers to have the new behaviour. Rich. -- Emerging Technologies, Red Hat - http://et.redhat.com/~rjones/ Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SL4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 03798903