This is a quick guide to mounting a qcow2 disk images on your host server. This is useful to reset passwords, edit files, or recover something without the virtual machine running.
Step 1 - Enable NBD on the Host
modprobe nbd max_part=8
Step 2 - Connect the QCOW2 as network block device
qemu-nbd --connect=/dev/nbd0 /var/lib/vz/images/100/vm-100-disk-1.qcow2
Step 3 - Find The Virtual Machine Partitions
fdisk /dev/nbd0 -l
Step 4 - Create the /dev/nbdXpY devices
According to [1] the partition device files don't appear just like that, you have to run
sudo partprobe /dev/nbd0
1 - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=824553
Step 5 - Mount the partition from the VM
mount /dev/nbd0p1 /mnt/somepoint/
Step 6 - After you're done, unmount and disconnect
umount /mnt/somepoint/
qemu-nbd --disconnect /dev/nbd0
rmmod nbd