efi_loader: fix dp_fill() for BLKMAP, HOST, VIRTIO

Do not assume that the preceding device path contains a single VenHW node.
Instead use the return value of dp_fill() which provides the address of the
next node.

Fixes: 23ad52ff ("efi_loader: device_path: support Sandbox's "host" devices")
Fixes: 19ecced7 ("efi_loader: device path for virtio block devices")
Fixes: 272ec6b4 ("efi_loader: device_path: support blkmap devices")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>