build: init qemu-based staging setup

I emphasize that it's a quote-on-quote staging setup, not really a
proper one yet but hopefully it'll be.

TODO: Maybe look into AWS MicroVM as an alternative to QEMU?

qemu.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+set -eux
+
+img=../qcow2/debian-12-backports-nocloud-amd64-20250210-2019.qcow2
+qemu-system-x86_64 \
+	-machine type=pc,accel=kvm \
+	-drive if=none,file="$img",id=hd0 \
+	-device virtio-blk,drive=hd0 \
+	-m 2G \
+	-enable-kvm \
+	-cpu max \
+	-device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 \
+	-nographic

setup_qemu.sh

@@ -0,0 +1,52 @@
+#!/bin/sh
+#
+# Adapted from https://wiki.debian.org/Arm64Qemu
+# * https://cdimage.debian.org/images/cloud/bookworm-backports/20250210-2019/
+
+set -eux
+
+skip="${1:-}"
+
+url=https://cdimage.debian.org/images/cloud/bookworm-backports/20250210-2019/debian-12-backports-nocloud-amd64-20250210-2019.qcow2
+img=../qcow2/debian-12-backports-nocloud-amd64-20250210-2019.qcow2
+
+mkdir -p ./.local
+
+if [ ! -f "$img" ]; then
+	cd -- ./.local
+	curl -LO -- "$url"
+	cd -- ..
+fi
+
+sudo apt install \
+	qemu-utils \
+	qemu-efi \
+	qemu-system-x86
+
+sudo modprobe nbd
+sudo qemu-nbd -c /dev/nbd0 "$img"
+if ! sudo mount /dev/nbd0p1 /mnt; then
+	<<-EOF cat -
+
+	I am unfortunately too lazy to actually understand why this fails
+	sometimes.
+
+	What's even more interesting is that it works if you execute again this
+	exact command outside of the script.
+
+	sudo mount /dev/nbd0p1 /mnt
+	"$0" skip # to resume this script
+
+	EOF
+	exit 0
+fi
+
+ssh-add -L >/mnt/home/debian/.ssh/authorized_keys
+sudo umount /mnt
+sudo qemu-nbd -d /dev/nbd0
+
+# qemu-system-aarch64 -m 2G -M virt -cpu max \
+#  -bios /usr/share/qemu-efi-aarch64/QEMU_EFI.fd \
+#  -drive if=none,file=debian-9.9.0-openstack-arm64.qcow2,id=hd0 -device virtio-blk-device,drive=hd0 \
+#  -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 \
+#  -nographic