How to make a roofs image for qemu using febootstrap

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 https://mirrors.163.com/centos/7/os/x86_64/

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?