scsi-disk: removable hard disks support load/eject

Support for the LOEJ bit of the START/STOP UNIT command right now is
limited to CD-ROMs.  This is wrong, since removable hard disks (in the
real world: SD card readers) also support it in pretty much the same way.

Without the LOEJ bit, START/STOP UNIT does nothing for all devices.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
