Mount QCOW2

Load Kernel module

modprobe nbd

Connect the image to NBD (Network Block Device) device and then mount that device/partition

qemu-nbd --connect=/dev/nbd0 /var/lib/vz/images/100/vm-100-disk-1.qcow2
mount /dev/nbd0p1 /mnt/somepoint/

When done unmount, disconnect, and if necessary unload the Kernel module.

umount /mnt/somepoint/
qemu-nbd --disconnect /dev/nbd0
rmmod nbd