dm: sandbox: Create a new HOST uclass

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>