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.

Backports commit 7408fb67c0f9403f6e40aecf97cf798fc14e2cd8 from qemu

qemu/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