The command lists the RPMs installed inside a Fedora / RHEL / CentOS or other RPM-based virtual machine. It does this by running the rpm -qa command directly inside the machine.

To use it you need to know the partition or LV inside the VM that contains the root filesystem, so for example: guest.img /dev/VolGroup00/LogVol00 | less

You can write a similar command for Debian-based virtual machines very easily.