efi_loader: provide agent_handle to efi_disk_add_dev()

In efi_disk_add_dev() we have to open protocols with BY_DRIVER and
BY_CHILD_CONTROLLER. Provide the handle of the EFI block driver. The actual
usage of the value will follow in a later patch.

Change function descriptions to Sphinx style.

Remove a TODO: tag.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>