qapi: Better error messages for bad enums

The previous commit demonstrated that the generator had several
flaws with less-than-perfect enums:
- an enum that listed the same string twice (or two variant
strings that map to the same C enumerator) ended up generating
an invalid C enum
- because the generator adds a _MAX terminator to each enum,
the use of an enum member 'max' can also cause this clash
- if an enum omits 'data', the generator left a python stack
trace rather than a graceful message
- an enum that used a non-array 'data' was silently accepted by
the parser
- an enum that used non-string members in the 'data' member
was silently accepted by the parser

Add check_enum to cover these situations, and update testcases
to match.  While valid .json files won't trigger any of these
cases, we might as well be nicer to developers that make a typo
while trying to add new QAPI code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

tests/qapi-schema/enum-missing-data.err

@@ -1,6 +1 @@
-Traceback (most recent call last):
-  File "tests/qapi-schema/test-qapi.py", line 19, in <module>
-    exprs = parse_schema(sys.argv[1])
-  File "scripts/qapi.py", line 334, in parse_schema
-    add_enum(expr['enum'], expr['data'])
-KeyError: 'data'
+tests/qapi-schema/enum-missing-data.json:2: Enum 'MyEnum' requires an array for 'data'