Reset of a forgotten password on a linux computer is quite easy when you have physical access to the machine.
The mandatory operation is to boot the machine in recovery mode. When running a VM it is quite difficult to reboot and get the prompt and change the boot option to a recovery kernel.
The solution is to go to the QNAP virtual machine console and click the Control-Alt-Delete icon to restart the machine.
Then you will be able to choose the boot option to recovery mode.
Once done you just have to type this command to mount the filesystem:
mount -rw -o remount /
to reset the forgotten password.