hotplug: more fixes for scsi disk hotplug.

Changes:
 - create common scsi_hot_add function for adding a scsi disk to
   a adapter.
 - Add sanity checks.  You can't drive_add disks to the VGA any more.
 - Ignore the unit value calculated by drive_init, add a comment
   explaining why.  Review the patch to find the details ;)

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
