qapi: improve reporting of unknown or missing keys

def check_known_keys(info, source, keys, required, optional):

    for key in keys:

        if key not in required and key not in optional:

            raise QAPISemError(info, "Unknown key '%s' in %s" % (key, source))

    for key in required:

        if key not in keys:

            raise QAPISemError(info, "Key '%s' is missing from %s"

                               % (key, source))

    def pprint(elems):

        return ', '.join("'" + e + "'" for e in sorted(elems))

    missing = set(required) - set(keys)

    if missing:

        raise QAPISemError(info, "Key%s %s %s missing from %s"

                           % ('s' if len(missing) > 1 else '', pprint(missing),

                              'are' if len(missing) > 1 else 'is', source))

    allowed = set(required + optional)

    unknown = set(keys) - allowed

    if unknown:

        raise QAPISemError(info, "Unknown key%s %s in %s\nValid keys are %s."

                           % ('s' if len(unknown) > 1 else '', pprint(unknown),

                              source, pprint(allowed)))

def check_keys(expr_elem, meta, required, optional=[]):

tests/qapi-schema/alternate-base.json:4: Unknown key 'base' in alternate 'Alt'

Valid keys are 'alternate', 'data', 'if'.

tests/qapi-schema/double-type.json:2: Unknown key 'command' in struct 'bar'

Valid keys are 'base', 'data', 'if', 'struct'.

tests/qapi-schema/unknown-expr-key.json:2: Unknown key 'bogus' in struct 'bar'

tests/qapi-schema/unknown-expr-key.json:2: Unknown keys 'bogus', 'phony' in struct 'bar'

Valid keys are 'base', 'data', 'if', 'struct'.

# we reject an expression with unknown top-level keys

{ 'struct': 'bar', 'data': { 'string': 'str'}, 'bogus': { } }

{ 'struct': 'bar', 'data': { 'string': 'str'}, 'bogus': { }, 'phony': { } }