efi_loader: implement EFI_DEVICE_PATH_TO_TEXT_PROTOCOL

ConvertPathToText is implemented for
* type 4    - media device path
* subtype 4 - file path

This is the kind of device path we hand out for block devices.

All other cases may be implemented later.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: fix whitespace]
Signed-off-by: Alexander Graf <agraf@suse.de>