qapi: Convert simple union ChardevBackend to flat one

qapi: Convert simple union ChardevBackend 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 ChardevBackend to
an equivalent flat one.  Adds some boilerplate to the schema, which is
a bit ugly, but a lot easier to maintain than the simple union
feature.

Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-8-armbru@redhat.com>
[Missing conditionals added]