qapi-types.py: Fix enum struct sizes on i686

Unlike other list types, enum wasn't adding any padding, which caused
a mismatch between the generated struct size and GenericList struct
size. More details in a678e26cbe89f7a27cbce794c2c2784571ee9d21

This crashed qemu if calling qmp query-tpm-types for example, which
upsets libvirt capabilities probing. Reproducer on i686:

(sleep 5; printf '{"execute":"qmp_capabilities"}\n{"execute":"query-tpm-types"}\n') | ./i386-softmmu/qemu-system-i386 -S -nodefaults -nographic -M none -qmp stdio

https://bugs.launchpad.net/qemu/+bug/1219207

Cc: qemu-stable@nongnu.org
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

scripts/qapi-types.py

@@ -51,7 +51,10 @@ def generate_fwd_enum_struct(name, members):
     return mcgen('''
 typedef struct %(name)sList
 {
-    %(name)s value;
+    union {
+        %(name)s value;
+        uint64_t padding;
+    };
     struct %(name)sList *next;
 } %(name)sList;
 ''',