block: Remove "options" indirection from blockdev-add

Now that QAPI supports boxed types, we can have unions at the top level
of a command, so let's put our real options directly there for
blockdev-add instead of having a single "options" dict that contains the
real arguments.

blockdev-add is still experimental and we already made substantial
changes to the API recently, so we're free to make changes like this
one, too.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>