If your image has UFS filesystems, qemu-nbd on Linux is useless. Here is what you do (example with a vsrx image):
qemu-img convert -f qcow2 -O raw junos-vsrx3-x86-64-18.4R1.8.qcow2 junos-vsrx3-x86-64-18.4R1.8.img
mdconfig -a -t vnode -f junos-vsrx3-x86-64-18.4R1.8.img -u 0
mount /dev/md0p2 /mnt
umount /mnt
mdconfig -d -u 0