The mirror has to be debian-ports, not debian, and many mirror sites do not
provide it. Following is aliyun mirror:
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/etc/apt/sources.list
---
deb https://mirrors.aliyun.com/debian-ports/ unstable main
deb https://mirrors.aliyun.com/debian-ports/ unreleased main
deb-src https://mirrors.aliyun.com/debian/ unstable main
GRUB in the image seems to use hard-coded path of /dev/* block file as the
root partition in the kernel command line rather than GUID, so if the hard disk
bus is changed in QEMU and the path is changed accordingly, the system can’t
boot on.
QEMU cli arguments -machine q35 enables AHCI and SATA, and is required for
official x86_64 image to boot. As for i386, I haven’t checked now.
There is a Deno script
written by me to help define and build QEMU cli arguments of VMs.