Is it set as bootable? I have seen some QCOWs not be read correctly by ovirt.
In that case I would take an OVA file and convert it manually to QCOW using the CLI,
sometimes the QCOW works only with IDE from what I have seen.
My way is somewhat long but usually works for me:
Upload to Disk page in GUI and attach to a premade VM -- If it fails use CLI:
CLI:
tar -xvf <image-name>.ova
it will extract to .vmdk files
OR if you have a VMDK already
qemu-img convert <image-name>-disk001.vmdk <image-name>.qcow2 -O qcow2
Once completed, you should verify that the file is successfully a QEMU QCOW2 Image.
file <image-name>.qcow2
You can then download over winscp and upload to the GUI Disks page and attach to a VM --
Sometimes if you have errors on boot you need to choose a different CPU or set HDD to IDE
or virtIO-Sata.