Drop unnecessary inclusions from headers.  Downgrade a few more to
exec/hwaddr.h.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190812052359.30071-17-armbru@redhat.com>