virtcontainers: make CreateDevice func private

CreateDevice() is only used by `NewDevices()` so we can make it private and
there's no need to export it.

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>

virtcontainers/device/api/interface.go

@@ -57,6 +57,5 @@ type Device interface {
 // DeviceManager can be used to create a new device, this can be used as single
 // device management object.
 type DeviceManager interface {
-	CreateDevice(devInfo config.DeviceInfo) Device
 	NewDevices(devInfos []config.DeviceInfo) ([]Device, error)
 }