qapi: Support downstream simple unions

Enhance the testsuite to cover downstream simple unions, including
when a union branch is a downstream name. Update the generator to
mangle the union names in the appropriate places.

Backports commit bb33729043ceda56b4068db13bdc17786ebd0ed0 from qemu

qemu/scripts/qapi-types.py

@@ -184,7 +184,7 @@ const int %(name)s_qtypes[QTYPE_MAX] = {
 
 def generate_union(expr, meta):
 
-    name = expr[meta]
+    name = c_name(expr[meta])
     typeinfo = expr['data']
 
     base = expr.get('base')

qemu/scripts/qapi-visit.py

@@ -256,9 +256,9 @@ def generate_visit_union(expr):
         disc_type = enum_define['enum_name']
     else:
         # There will always be a discriminator in the C switch code, by default
-        # it is an enum type generated silently as "'%sKind' % (name)"
-        ret = generate_visit_enum('%sKind' % name, members.keys())
-        disc_type = '%sKind' % (name)
+        # it is an enum type generated silently
+        ret = generate_visit_enum(name + 'Kind', members.keys())
+        disc_type = c_name(name) + 'Kind'
 
     if base:
         assert discriminator
@@ -282,7 +282,7 @@ void visit_type_%(name)s(Visitor *m, %(name)s **obj, const char *name, Error **e
     }
     if (*obj) {
 ''',
-                 name=name)
+                 name=c_name(name))
 
     if base:
         ret += mcgen('''