[Libvir] 2/4 QEMU driver

This patch implements the QEMU driver part of the change. We run dnsmasq with the extra --dhcp-hostsfile=... parameter, and then include functions to read and write this file. When the file changes we send SIGHUP to dnsmasq. 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