efi_loader: VenMedia device path node

The Load File2 protocol exposes a device path with a VenMedia() node. Hence
our implementation of the device path to text protocol should support this
node.

Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>