devices: use device manager to manage all devices

Fixes #50

Previously the devices are created with device manager and laterly
attached to hypervisor with "device.Attach()", this could work, but
there's no way to remember the reference count for every device, which
means if we plug one device to hypervisor twice, it's truly inserted
twice, but actually we only need to insert once but use it in many
places.

Use device manager as a consolidated entrypoint of device management can
give us a way to handle many "references" to single device, because it
can save all devices and remember it's use count.

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

virtcontainers/device/api/interface.go

@@ -27,6 +27,7 @@ func DeviceLogger() *logrus.Entry {
 // DeviceReceiver is an interface used for accepting devices
 // a device should be attached/added/plugged to a DeviceReceiver
 type DeviceReceiver interface {
+	// these are for hotplug/hot-unplug devices to/from hypervisor
 	HotplugAddDevice(Device, config.DeviceType) error
 	HotplugRemoveDevice(Device, config.DeviceType) error
 
@@ -51,11 +52,29 @@ type VhostUserDevice interface {
 type Device interface {
 	Attach(DeviceReceiver) error
 	Detach(DeviceReceiver) error
+	// ID returns device identifier
+	DeviceID() string
+	// DeviceType indicates which kind of device it is
+	// e.g. block, vfio or vhost user
 	DeviceType() config.DeviceType
+	// GetDeviceInfo returns device information that the device is created based on
+	GetDeviceInfo() *config.DeviceInfo
+	// GetDeviceDrive returns device specific data used for hotplugging by hypervisor
+	// Caller could cast the return value to device specific struct
+	// e.g. Block device returns *config.BlockDrive and
+	// vfio device returns *config.VFIODrive
+	GetDeviceDrive() interface{}
+	// IsAttached checks if the device is attached
+	IsAttached() bool
 }
 
 // DeviceManager can be used to create a new device, this can be used as single
 // device management object.
 type DeviceManager interface {
-	NewDevices(devInfos []config.DeviceInfo) ([]Device, error)
+	NewDevice(config.DeviceInfo) (Device, error)
+	AttachDevice(string, DeviceReceiver) error
+	DetachDevice(string, DeviceReceiver) error
+	IsDeviceAttached(string) bool
+	GetDeviceByID(string) Device
+	GetAllDevices() []Device
 }