drive: allow rerror, werror and readonly for if=none

When creating guest disks the qdev way using ...

  -drive if=none,id=$name,args
  -device $driver,drive=$name

it is not possible to specify rerror, werror and readonly arguments
for drive as drive_init allows/blocks them based on the interface (if=)
specified and none isn't white-listed there.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>