hw/riscv: virt: pass random seed to fdt

hw/riscv: virt: pass random seed to fdt

If the FDT contains /chosen/rng-seed, then the Linux RNG will use it to
initialize early. Set this using the usual guest random number
generation function. This is confirmed to successfully initialize the
RNG on Linux 5.19-rc2.

Cc: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20220613115810.178210-1-Jason@zx2c4.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>