drives: add CD-ROM disk images as read-only drives (RHBZ#563450).

The current add_cdrom way basically appends a new raw "-cdrom /path"
parameter to the qemu invocation (even when using libvirt as backend),
hence such images are seen as "CD-ROM drives" inside the appliance.
However, there is no need for such particular behaviour, as they need to
be handled as normal (read-only) drives.

Adding CD-ROM disk images as drives also changes the device names used
for them inside the appliance from /dev/srN to the usual e.g. /dev/sdX.

These changes fix different issues:
- it is possible to start guestfish without adding disks with -a, then
  just add-cdrom and run
- list-devices does not cause guestfsd to crash when sorting the list
  of devices (exposed by the test case in RHBZ#563450)
- the result of list-devices now reflects the order images were added
  (RHBZ#563450)

add_cdrom is still deprecated, but now in favour of add_drive_ro
(instead of add_drive), with its documentation reflecting that.

Add two small regression tests for the fixes described above.