qapi: Improve 'include' error message

Use of '"...%s" % include' to print non-strings can lead to
ugly messages, such as this (if the .json change is applied
without the qapi.py change):
 Expected a file name (string), got: OrderedDict()

Better is to just omit the actual non-string value in the
message.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1443565276-4535-3-git-send-email-eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

scripts/qapi.py

@@ -132,8 +132,7 @@ class QAPISchemaParser(object):
                 include = expr["include"]
                 if not isinstance(include, str):
                     raise QAPIExprError(expr_info,
-                                        'Expected a file name (string), got: %s'
-                                        % include)
+                                        "Value of 'include' must be a string")
                 incl_abs_fname = os.path.join(os.path.dirname(abs_fname),
                                               include)
                 # catch inclusion cycle

tests/qapi-schema/include-non-file.err

-tests/qapi-schema/include-non-file.json:1: Expected a file name (string), got: ['foo', 'bar']
+tests/qapi-schema/include-non-file.json:1: Value of 'include' must be a string

tests/qapi-schema/include-non-file.json

-{ 'include': [ 'foo', 'bar' ] }
+{ 'include': {} }