qapi: Enable enum member introspection to show more than name

The next commit will add feature flags to enum members.  There's a
problem, though: query-qmp-schema shows an enum type's members as an
array of member names (SchemaInfoEnum member @values).  If it showed
an array of objects with a name member, we could simply add more
members to these objects.  Since it's just strings, we can't.

I can see three ways to correct this design mistake:

1. Do it the way we should have done it, plus compatibility goo.

   We want a ['SchemaInfoEnumMember'] member in SchemaInfoEnum.  Since
   changing @values would be a compatibility break, add a new member
   @members instead.

   @values is now redundant.  In my testing, output of
   qemu-system-x86_64's query-qmp-schema grows by 11% (18.5KiB).

   We can deprecate @values now and drop it later.  This will break
   outmoded clients.  Well-behaved clients such as libvirt are
   expected to break cleanly.

2. Like 1, but omit "boring" elements of @member, and empty @member.

   @values does not become redundant.  @members augments it.  Somewhat
   cumbersome, but output of query-qmp-schema grows only as we make
   enum members non-boring.

   There is nothing to deprecate here.

3. Versioned query-qmp-schema.

   query-qmp-schema provides either @values or @members.  The QMP
   client can select which version it wants.  There is no redundant
   output.

   We can deprecate old versions and eventually drop them.  This will
   break outmoded clients.  Breaking cleanly is easier than for 1.

   While 1 and 2 operate within the common rules for compatible
   evolution apply (section "Compatibility considerations" in
   docs/devel/qapi-code-gen.rst), 3 bypasses them.  Attractive when
   operating within the rules is just too awkward.  Not the case here.

This commit implements 1.  Libvirt developers prefer it.

Deprecate @values in favour of @members.  Since query-qmp-schema
compatibility is pretty fundamental for management applications, an
extended grace period is advised.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Message-Id: <20211025042405.3762351-2-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

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