drive_init() doesn't permit rerror for if=scsi, but that's worthless:
we get it via if=none and -device.

Moreover, scsi-generic doesn't support werror.  Since drive_init()
doesn't catch that, option werror was silently ignored even with
if=scsi.

Wart: unlike drive_init(), we don't reject the default action when
it's explicitly specified.  That's because we can't distinguish "no
rerror option" from "rerror=report", or "no werror" from
"rerror=enospc".  Left for another day.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>