Replace the simple list sugar form with a recursive structure that will
accept other operators in the following commits (all, any or not).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210804083105.97531-7-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Accidental code motion undone.  Degenerate :forms: comment dropped.
Helper _check_if() moved.  Error messages tweaked.  ui.json updated.
Accidental changes to qapi-schema-test.json dropped.]
Signed-off-by: Markus Armbruster <armbru@redhat.com>