qapi: Convert simple union SocketAddressLegacy to flat one

Simple unions predate flat unions.  Having both complicates the QAPI
schema language and the QAPI generator.  We haven't been using simple
unions in new code for a long time, because they are less flexible and
somewhat awkward on the wire.

To prepare for their removal, convert simple union SocketAddressLegacy
to an equivalent flat one, with existing enum SocketAddressType
replacing implicit enum type SocketAddressLegacyKind.  Adds some
boilerplate to the schema, which is a bit ugly, but a lot easier to
maintain than the simple union feature.

Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-9-armbru@redhat.com>