Hi Bintian,
As Chistopher said in this post, did you add ext4 in the menuconfig ?
things to check after menuconfig in your .config
CONFIG_BLK_DEV=y
CONFIG_VIRTIO_BLK=y <<=== need to be added menuconfig missed(*)
CONFIG_VIRTIO=y
CONFIG_VIRTIO_BALLOON=y
CONFIG_VIRTIO_MMIO=y
CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES=y
(*) may be I goofed with menuconfig, so I added a posteriori manually
in .config and forced y
The make the kernel
rebuild your linux-system-foundation.axf and give it to the Foundation.
Should do the trick I guess