[libvirt] [PATCH]: Fix Qemu CD-ROM with no source

The new generic domain re-factor introduced a small regression into the drive handling code. In particular, if you had a section of XML like: <disk type='file' device='cdrom'> <target dev='hdc' bus='ide'/> <readonly/> </disk> This used to work with older libvirt, but now fails. This is because we are actually passing the literal string (null) to the qemu command-line, which qemu barfs on. This patch fixes it up by making it blank, which allows qemu to continue on it's merry way. Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt;