qapi-dealloc: Reduce use outside of generated code

No need to roll our own use of the dealloc visitors when we can
just directly use the qapi_free_FOO() functions that do what we
want in one line.

In net.c, inline net_visit() into its remaining lone caller.

After this patch, test-visitor-serialization.c is the only
non-generated file that needs to use a dealloc visitor, because
it is testing low level aspects of the visitor interface.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1456262075-3311-2-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>