Sandbox supports block devices which can access files on the host machine.
At present there is no uclass for this. The devices are attached to the
root devic. The block-device type is therefore set to UCLASS_ROOT which
is confusing.
Block devices should be attached to a 'media' device instead, something
which handles access to the actual media and provides the block driver
for the block device.
Create a new uclass to handle this. It supports two operations, to attach
and detach a file on the host machine.
For now this is not fully plumbed in.
Signed-off-by: Simon Glass <sjg@chromium.org>
