efi_loader: correct HandleProtocol()

The UEFI specification requires that when a protocol is opened via
HandleProtocol() the agent handle is the image handle of the EFI firmware
(see chapter on EFI_BOOT_SERVICES.OpenProtocol()).

Let efi_handle_protocol() pass efi_root as agent handle to
efi_open_protocol().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>