My question is how to convert a pre-created OpenVZ OS template which are in tar.gz format (such as these) to an image file in order to be able boot it with other virtualization solutions such as QEMU or VirtualBox.

In order to achieve this, I made an empty image file, I partitioned it, and made two partition, a primary partition and a extended partition for swap. I made the first partition ext3 (0x83) and the other one swap (0x82). Then I made the first one bootable, and copied the content of tar.gz to the first partition. But when I try to boot, it hangs at the first stage of booting.