block: Remove "options" indirection from blockdev-add

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 <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Max Reitz <[email protected]>