build: avoid type-punning in vbox

Commit 78345c68 makes at least gcc 4.1.2 on RHEL 5 complain:

cc1: warnings being treated as errors
In file included from vbox/vbox_V4_0.c:13:
vbox/vbox_tmpl.c: In function 'vboxDomainUndefineFlags':
vbox/vbox_tmpl.c:5298: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

* src/vbox/vbox_tmpl.c (vboxDomainUndefineFlags): Use union to
avoid compiler warning.

src/vbox/vbox_tmpl.c

@@ -5294,8 +5294,11 @@ vboxDomainUndefineFlags(virDomainPtr dom, unsigned int flags)
 
         ((IMachine_Delete)machine->vtbl->Delete)(machine, &safeArray, &progress);
 # else
-        vboxArray array = VBOX_ARRAY_INITIALIZER;
-        machine->vtbl->Delete(machine, 0, (IMedium**)&array, &progress);
+        union {
+            vboxArray array;
+            IMedium *medium;
+        } u = { .array = VBOX_ARRAY_INITIALIZER };
+        machine->vtbl->Delete(machine, 0, &u.medium, &progress);
 # endif
         if (progress != NULL) {
             progress->vtbl->WaitForCompletion(progress, -1);