vendor: Update govmm to add vfio mediated device support on root bus

In addition to supporting hotplug for VFIO mediated device on PCI bridge,
this patch adds hotplug functionality on root bus.

When parameter bus and addr are set to be empty, the system will pick up
an empty slot on root bus.

Fixes #542

Signed-off-by: Zhao Xinda <xinda.zhao@intel.com>

vendor/github.com/intel/govmm/qemu/qmp.go

@@ -946,11 +946,13 @@ func (q *QMP) ExecutePCIVFIOMediatedDeviceAdd(ctx context.Context, devID, sysfsd
 		"id":       devID,
 		"driver":   "vfio-pci",
 		"sysfsdev": sysfsdev,
-		"addr":     addr,
 	}
 	if bus != "" {
 		args["bus"] = bus
 	}
+	if addr != "" {
+		args["addr"] = addr
+	}
 	return q.executeCommand(ctx, "device_add", args, nil)
 }