dm: virtio-input: implement virtio_input_init

The following are done in virtio_input_init:
- parse the command line to get the path of host evdev
- parse the command line to get the optional serial string
- calloc struct virtio_input and initialize it
- call virtio framework APIs to initialize virtio PCI

Signed-off-by: Jian Jun Chen <jian.jun.chen@intel.com>
Reviewed-by: Zhao Yakui <yakui.zhao@intel.com>
Reviewed-by: Hao Li <hao.l.li@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

devicemodel/include/virtio.h

@@ -215,6 +215,7 @@ struct vring_used {
 #define	VIRTIO_TYPE_RPMSG	7
 #define	VIRTIO_TYPE_SCSI	8
 #define	VIRTIO_TYPE_9P		9
+#define	VIRTIO_TYPE_INPUT	18
 
 /*
  * ACRN virtio device types