qemu-riscv: enable VIRTIO_PCI

libvirt v.5.3.0 with QEMU 4.0.0 or above uses PCI automatically and
thus devices (network, storage, etc) are connected via PCI.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>