[Libvir] Please check my autoconf libvirt.m4

Attached is an autoconf snippet which allows you to check that the libvirt library is installed (not necessarily development headers though) and check the version number. If any autoconf experts (hello, Mark) would like to check it makes sense, that'd be great. 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