qapi: Change data type of the FOO_lookup generated for enum FOO

Currently, a FOO_lookup is an array of strings terminated by a NULL
sentinel.

A future patch will generate enums with "holes".  NULL-termination
will cease to work then.

To prepare for that, store the length in the FOO_lookup by wrapping it
in a struct and adding a member for the length.

The sentinel will be dropped next.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170822132255.23945-13-marcandre.lureau@redhat.com>
[Basically redone]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-16-git-send-email-armbru@redhat.com>
[Rebased]
diff --git a/qemu-nbd.c b/qemu-nbd.c
index a97f3f4..d75ca51 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -638,7 +638,7 @@
             break;
         case QEMU_NBD_OPT_DETECT_ZEROES:
             detect_zeroes =
-                qapi_enum_parse(BlockdevDetectZeroesOptions_lookup,
+                qapi_enum_parse(&BlockdevDetectZeroesOptions_lookup,
                                 optarg,
                                 BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF,
                                 &local_err);