We have two new machine options "aia" and "aia-guests" available
for the RISC-V virt machine so let's document these options.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-Id: <20220220085526.808674-5-anup@brainfault.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>