libcamera: v4l2_device: Add setFd()

Provide a means for V4L2 device instances to set the fd_ explicitly.
This allows for V4L2Devices to operate when they must use an external
open() call.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/include/v4l2_device.h

@@ -35,6 +35,7 @@ protected:
 	~V4L2Device();
 
 	int open(unsigned int flags);
+	int setFd(int fd);
 
 	int ioctl(unsigned long request, void *argp);