cmd-buildinitramfs-fast: specify backing image format
Otherwise `qemu-img create` will just refuse to operate now.
@@ -59,7 +59,7 @@ test -f "${initramfs}" || fatal "missing ${initramfs}"
-D . | gzip -1) >>"${initramfs}"
fastbuild_qemu="fastbuildinitrd-${name}-qemu.qcow2"
-qemu-img create -f qcow2 -o backing_file="${previous_builddir}/${previous_qemu}" "${fastbuild_qemu}" 20G
+qemu-img create -F qcow2 -f qcow2 -o backing_file="${previous_builddir}/${previous_qemu}" "${fastbuild_qemu}" 20G
coreos_gf_run_mount ro "${fastbuild_qemu}"
coreos_gf remount /boot rw:true
