qemu: Don't delete USB device on failed qemuPrepareHostdevUSBDevices

If qemuPrepareHostdevUSBDevices fail it will roll back devices added
to the driver list of used devices. However, if it may fail because
the device is being used already. But then again - with roll back.
Therefore don't try to remove a usb device manually if the function
fail. Although, we want to remove the device if any operation
performed afterwards fail.

src/qemu/qemu_hotplug.c

@@ -1173,8 +1173,10 @@ int qemuDomainAttachHostDevice(struct qemud_driver *driver,
             goto cleanup;
         }
 
-        if (qemuPrepareHostdevUSBDevices(driver, vm->def->name, list) < 0)
+        if (qemuPrepareHostdevUSBDevices(driver, vm->def->name, list) < 0) {
+            usb = NULL;
             goto cleanup;
+        }
 
         usbDeviceListSteal(list, usb);
     }