qapi/qom: Add ObjectOptions for throttle-group

This adds a QAPI schema for the properties of the throttle-group object.

The only purpose of the x-* properties is to make the nested options in
'limits' available for a command line parser that doesn't support
structs. Any parser that will use the QAPI schema will supports structs,
though, so they will not be needed in the schema in the future.

To keep the conversion straightforward, add them to the schema anyway.
We can then remove the options and adjust documentation, test cases etc.
in a separate patch.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>