qapi: Use anonymous bases in QMP flat unions

qapi: Use anonymous bases in QMP flat unions

Now that the generator supports it, we might as well use an
anonymous base rather than breaking out a single-use Base
structure, for all three of our current QMP flat unions.

Oddly enough, this change does not affect the resulting
introspection output (because we already inline the members of
a base type into an object, and had no independent use of the
base type reachable from a command).

The case_whitelist now has to list the name of an implicit
type; which is not too bad (consider it a feature if it makes
it harder for developers to make the whitelist grow :)

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <1458254921[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>