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

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 <[email protected]>
Message-Id: <20170822132255[email protected]>
[Basically redone]
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <1503564371[email protected]>
[Rebased]