Be more strict when editing /etc/libvirt/qemu.conf

We could match more lines than what we wanted.
Do not always mark qemu.conf has changed

Chef::Util::FileEdit is not very clever and can replace a line with the
exact same content. So we cannot really trust the fact that it was
edited. Instead, rely on md5sums.
Merge pull request #195 from vuntz/strict-edit

Be more strict when editing /etc/libvirt/qemu.conf