qapi: Generalize struct member policy checking

The generated visitor functions call visit_deprecated_accept() and
visit_deprecated() when visiting a struct member with special feature
flag 'deprecated'.  This makes the feature flag visible to the actual
visitors.  I want to make feature flag 'unstable' visible there as
well, so I can add policy for it.

To let me make it visible, replace these functions by
visit_policy_reject() and visit_policy_skip(), which take the member's
special features as an argument.  Note that the new functions have the
opposite sense, i.e. the return value flips.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211028102520.747396-6-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[Unbreak forward visitor]