Fix crash on usb-serial hotplug

For domains with no USB address cache, we should not attempt
to generate a USB address.

https://bugzilla.redhat.com/show_bug.cgi?id=1387665
(cherry picked from commit 00c5386c)

src/qemu/qemu_hotplug.c

@@ -1608,7 +1608,8 @@ qemuDomainAttachChrDeviceAssignAddr(virDomainDefPtr def,
             goto cleanup;
         ret = 1;
 
-    } else if (chr->deviceType == VIR_DOMAIN_CHR_DEVICE_TYPE_SERIAL &&
+    } else if (priv->usbaddrs &&
+               chr->deviceType == VIR_DOMAIN_CHR_DEVICE_TYPE_SERIAL &&
                chr->targetType == VIR_DOMAIN_CHR_SERIAL_TARGET_TYPE_USB) {
         if (virDomainUSBAddressEnsure(priv->usbaddrs, &chr->info) < 0)
             goto cleanup;