Prevent CD-ROM media eject while device is locked

Section 10.8.25 ("START/STOP UNIT Command") of SFF-8020i states that
if the device is locked we should refuse to eject if the device is
locked.

ASC_MEDIA_REMOVAL_PREVENTED is the appropriate return in this case.

In order to stop itself from ejecting the media it is running from,
Fedora's installer (anaconda) requires the CDROMEJECT ioctl() to fail
if the drive has been previously locked.

See also https://bugzilla.redhat.com/501412

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
