rbd: Clean up runtime_opts, fix -drive to reject filename

rbd: Clean up runtime_opts, fix -drive to reject filename

runtime_opts is used for three different purposes:

* qemu_rbd_open() uses it to accept options it recognizes, such as
  "pool" and "image".  Other .bdrv_open() methods do it similarly.

* qemu_rbd_open() accepts additional list-valued options
  auth-supported and server, with the help of qemu_rbd_array_opts().
  The list elements are again dictionaries.  qemu_rbd_array_opts()
  uses runtime_opts to accept their members.  Thus, runtime_opts
  contains recognized sub-sub-options "auth", "host", "port" in
  addition to recognized options.  No other block driver does that.

* qemu_rbd_create() uses it to convert the QDict produced by
  qemu_rbd_parse_filename() to QemuOpts.  No other block driver does
  that.  The keys produced by qemu_rbd_parse_filename() are "pool",
  "image", "snapshot", "conf", "user" and "keyvalue-pairs".
  qemu_rbd_open() accepts these, so no additional ones here.

This is a confusing mess.  Dates back to commit 0f9d252.  First step
to clean it up is documenting runtime_opts.desc[]:

* Reorder entries to match the QAPI schema, like we do in other block
  drivers.

* Document why the schema's "server" and "auth-supported" aren't in
  .desc[].

* Document why "keyvalue-pairs", "host", "port" and "auth" are in
  .desc[], but not the schema.

* Delete "filename", because none of the three users actually uses it.
  This fixes -drive to reject parameter filename instead of silently
  ignoring it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1490691368-32099-7-git-send-email-armbru@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>