efi_loader: reformat efi_disk_add_dev()

Make it clearer why InstallMultipleProtocolInterfaces is invoked with two
NULLs:

* rename guid to esp_guid
* put protocol GUIDs and the related interfaces on same lines
* add comment

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>