drive cleanup fixes.

drive cleanup fixes.

Changes:
  * drive_uninit() wants a DriveInfo now.
  * drive_uninit() also calls bdrv_delete(),
    so callers don't need to do that.
  * drive_uninit() calls are moved over to the ->exit()
    callbacks, destroy_bdrvs() is zapped.
  * setting bdrv->private is not needed any more as the
    only user (destroy_bdrvs) is gone.
  * usb-storage needs no drive_uninit, scsi-disk will
    handle that.

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