qapi: Move context-free checking to the proper place
QAPISchemaCommand.check() and QAPISchemaEvent().check() check 'data'
is present when 'boxed': true. That's context-free. Move to
check_command() and check_event().
Tweak the error message while there.
check_exprs() & friends now check exactly what qapi-code-gen.txt calls
the second layer of syntax.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <
20190927134639.4284-18-armbru@redhat.com>