imply VIRTIO_VGA for the virt machine, this fixes the following error
when specifying `-vga virtio` as a command line argument:

qemu-system-riscv64: Virtio VGA not available

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 7ac26fafee8bd59d2a0640f3233f8ad1ab270e1e.1617367317.git.alistair.francis@wdc.com