qemu: fix use after free

Detected by Coverity.

* src/qemu/qemu_hotplug.c (qemuDomainAttachHostDevice): Avoid
double free of usb on failure.

src/qemu/qemu_hotplug.c

@@ -1170,6 +1170,7 @@ int qemuDomainAttachHostDevice(struct qemud_driver *driver,
 
         if (usbDeviceListAdd(list, usb) < 0) {
             usbFreeDevice(usb);
+            usb = NULL:
             goto cleanup;
         }