qapi: convert BlockdevOptions to use enum discriminator

After this patch, hidden enum type BlockdevOptionsKind will not
be generated, and other API can use enum BlockdevDriver.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>