extract the disk image

tar -xvf junos-vsrx-12.1X47-D20.7-domestic.ova

convert VMDK to QCOW2 format

qemu-img convert -O qcow2 junos-vsrx-12.1X47-D20.7-domestic-disk1.vmdk junos-vsrx-12.1X47-D20.7-domestic.qcow2