With 0.5.0 release of `rust-hypervisor-firmware`, `aarch64` binary were
added to assets, which causes the `FW_URL` to have multiple download url
separated by a white space, thus our integration tests would fail.
Constrain `FW_URL` to `hypervisor-fw` to resolve this.
Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>