efi_loader: device path for virtio block devices

The UEFI specification does not define a device sub-type for virtio.
Let's use a vendor hardware node here.

This avoids creation of two handles with the same device path indicating
our root node.

Reported-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>