The new generic domain re-factor introduced a small regression into the drive

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@redhat.com>