qapi: Don't let implicit enum MAX member collide

Now that we guarantee the user doesn't have any enum values
beginning with a single underscore, we can use that for our
own purposes.  Renaming ENUM_MAX to ENUM__MAX makes it obvious
that the sentinel is generated.

This patch was mostly generated by applying a temporary patch:

|diff --git a/scripts/qapi.py b/scripts/qapi.py
|index e6d014b..b862ec9 100644
|--- a/scripts/qapi.py
|+++ b/scripts/qapi.py
|@@ -1570,6 +1570,7 @@ const char *const %(c_name)s_lookup[] = {
|     max_index = c_enum_const(name, 'MAX', prefix)
|     ret += mcgen('''
|     [%(max_index)s] = NULL,
|+// %(max_index)s
| };
| ''',
|                max_index=max_index)

then running:

$ cat qapi-{types,event}.c tests/test-qapi-types.c |
    sed -n 's,^// \(.*\)MAX,s|\1MAX|\1_MAX|g,p' > list
$ git grep -l _MAX | xargs sed -i -f list

The only things not generated are the changes in scripts/qapi.py.

Rejecting enum members named 'MAX' is now useless, and will be dropped
in the next patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-23-git-send-email-eblake@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
[Rebased to current master, commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>