virttest.qemu_devices: Fix boot=on not being passed to RHEL5 qemu

On RHEL5, we were not passing boot=on properly to the
qemu-kvm binary. Fix the heuristics to detect whether
the binary uses this ancient syntax to define virtio
disks as bootable.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>