VFS: Unable to mount root fs on unknown-block(0,0)

[泰宇周]

I tried to use febootstrap to make a roofs image for qemu-kvm.

I frist generated a roofs with command

    febootstrap -i coreutils -i vim centos79 centos_7.9 
[url]https://mirrors.163.com/centos/7/os/x86_64/[/url]

After that, I got a directory as following

    > tree centos_7.9/ -L 1
    centos_7.9/
    ├── bin -> usr/bin
    ├── boot
    ├── dev
    ├── etc
    ├── home
    ├── lib -> usr/lib
    ├── lib64 -> usr/lib64
    ├── media
    ├── mnt
    ├── opt
    ├── proc
    ├── root
    ├── run
    ├── sbin -> usr/sbin
    ├── srv
    ├── sys
    ├── tmp
    ├── usr
    └── var

Then, I created a qemu image and copied all file from centos_7.9 to the image.

    qemu-img create centos79.img 1g
    mkfs.ext4 centos79.img
    mkdir mnt_dir
    sudo mount -o loop centos79.img mnt_dir
    cp centos_7.9/* mnt_dir
    umount mnt_dir

However, when I run 

    qemu-system-x86_64 -kernel /boot/vmlinuz-`uname -r` \
        -hda=centos79.img \
        -append "root=/dev/sda console=ttyS0" \
        -enable-kvm -nographic

or 

    qemu-system-x86_64 -kernel /boot/vmlinuz-`uname -r` \
        -drive file=centos79.img \
        -append "root=/dev/hda console=ttyS0" \
        -enable-kvm -nographic


It reported that

    [    1.766919] Kernel panic - not syncing: VFS: Unable to mount root fs on 
unknown-block(0,0)
    [    1.767906] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.14.0-3.1.0.kwai+ 
#10
    [    1.767906] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
0.5.1 01/01/2011
    [    1.767906] Call Trace:
    [    1.767906]  dump_stack_lvl+0x34/0x48
    [    1.767906]  panic+0xfb/0x2cd
    [    1.767906]  mount_block_root+0x28c/0x29f
    [    1.767906]  prepare_namespace+0x13b/0x16a
    [    1.767906]  kernel_init_freeable+0x17f/0x1a4
    [    1.767906]  ? rest_init+0xc0/0xc0
    [    1.767906]  kernel_init+0x16/0x130
    [    1.767906]  ret_from_fork+0x22/0x30
    [    1.767906] Kernel Offset: disabled
    [    1.767906] ---[ end Kernel panic - not syncing: VFS: Unable to mount 
root fs on unknown-block(0,0) ]---


What should I do to fix this?