An ova file is just a tarball containing an ovf file and a vmdk file. The ovf file is a text-based configuration format, so you can get a basic idea of the config you'd need for qemu. Then the vmdk can be converted with qemu-img.

I used the following qemu-img command:

    qemu-img convert -O qcow2 archiveteam-warrior-v3-20171013-disk001.vmdk archiveteam-warrior-v3-20171013-disk001.qcow2

I use the following to run the VM (I gave it some more memory because I have plenty to space):

    qemu-system-x86_64 -m 1024 archiveteam-warrior-v3-20171013-disk001.qcow2

I think they were doing some kind of port forwarding, but I didn't bother, and I just access the web interface using the VM's IP (you can hit alt-right arrow to go to a login prompt and log in as root then run "ip a" to see the IP).