VMWare Fusion 5 can set the CD-ROM's device name to be 'auto detect' when
using the physical drive via 'cdrom-raw' device type. VMWare will then
connect to first available host CD-ROM to the virtual machine upon start
up according to VMWare documentation. If no device is available, it
appears that the device will remain disconnected.

To better model this a CD-ROM that is marked as "auto detect" when in
the off state would be modeled as the following with this patch:
  <disk type='block' device='lun'>
    <source startupPolicy='optional'/>
    <target dev='hda' bus='ide'/>
    <address type='drive' controller='0' bus='0' target='0' unit='0'/>
  </disk>

Once the domain transitions to the powered on state, libvirt can
populate the remaining source data with what is connected, if anything.
However future power cycles, the domain may not always start with that
device attached.