efi_loader: device paths for special block devices

The UEFI specification does not provide node types matching UCLASS_BLKMAP,
UCLASS_HOST, UCLASS_VIRTIO block devices.

The current implementation uses VenHw() nodes with uclass specific GUIDs
and a single byte for the device number appended. This leads to unaligned
integers in succeeding device path nodes.

The current implementation fails to create unique device paths for block
devices based on other uclasses like UCLASS_PVBLOCK.

Let's use a VenHw() node with the U-Boot GUID with a length dividable by
four and encoding blkdesc->uclass_id as well as  blkdesc->devnum.

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