qapi script: support enum type as discriminator in union

qapi script: support enum type as discriminator in union

By default, any union will automatically generate a enum type as
"[UnionName]Kind" in C code, and it is duplicated when the discriminator
is specified as a pre-defined enum type in schema. After this patch,
the pre-defined enum type will be really used as the switch case
condition in generated C code, if discriminator is an enum field.

Signed-off-by: Wenchao Xia <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Luiz Capitulino <[email protected]>