tests/pxe: Check virtio-net-ccw on s390x

Now that we've got a firmware that can do TFTP booting on s390x (i.e.
the pc-bios/s390-netboot.img), we can enable the PXE tester for this
architecture, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <1502431076-22849-3-git-send-email-thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>