qemu: command: Drop formatting of 'media=cdrom' from -drive

qemu: command: Drop formatting of 'media=cdrom' from -drive

For SCSI, IDE, and AHCI cdroms the appropriate device types which select
the correct media are used. In qemu there's one other code path that
looks at -drive media=cdrom in the XEN pv code. Thankfully we don't
support it with qemu (see qemuBuildDiskDeviceStr). All other devices
ignore it as the comment states, thus we can drop that code.

The test fallout is expectedly only in the test added for uncommon cdrom
types.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>