build: avoid uninitialized variable warning

* src/vbox/vbox_tmpl.c (vboxAttachDrives): Capture return value.

src/vbox/vbox_tmpl.c

@@ -3795,7 +3795,7 @@ vboxAttachDrives(virDomainDefPtr def, vboxGlobalData *data, IMachine *machine)
                 VBOX_UTF8_TO_UTF16("", &mediumEmpty);
 
                 if (def->disks[i]->device == VIR_DOMAIN_DISK_DEVICE_DISK) {
-                    data->vboxObj->vtbl->OpenHardDisk(data->vboxObj,
+                    rc = data->vboxObj->vtbl->OpenHardDisk(data->vboxObj,
                                                            mediumFileUtf16,
                                                            AccessMode_ReadWrite,
                                                            false,
@@ -3805,16 +3805,18 @@ vboxAttachDrives(virDomainDefPtr def, vboxGlobalData *data, IMachine *machine)
                                                            &medium);
                 } else if (def->disks[i]->device ==
                            VIR_DOMAIN_DISK_DEVICE_CDROM) {
-                    data->vboxObj->vtbl->OpenDVDImage(data->vboxObj,
+                    rc = data->vboxObj->vtbl->OpenDVDImage(data->vboxObj,
                                                            mediumFileUtf16,
                                                            mediumEmpty,
                                                            &medium);
                 } else if (def->disks[i]->device ==
                            VIR_DOMAIN_DISK_DEVICE_FLOPPY) {
-                    data->vboxObj->vtbl->OpenFloppyImage(data->vboxObj,
+                    rc = data->vboxObj->vtbl->OpenFloppyImage(data->vboxObj,
                                                               mediumFileUtf16,
                                                               mediumEmpty,
                                                               &medium);
+                } else {
+                    rc = 0;
                 }
 
                 VBOX_UTF16_FREE(mediumEmpty);