builder/virtualbox/iso: fix incorrect printf verb type

Fixes the following vet report:

builder/virtualbox/iso/builder_test.go:86: arg b.config.DiskSize for printf verb %s of wrong type: uint

builder/virtualbox/iso/builder_test.go

@@ -83,7 +83,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 	}
 
 	if b.config.DiskSize != 60000 {
-		t.Fatalf("bad size: %s", b.config.DiskSize)
+		t.Fatalf("bad size: %d", b.config.DiskSize)
 	}
 }