docs: kvm: riscv: document EBUSY in KVM_SET_ONE_REG

The EBUSY errno is being used for KVM_SET_ONE_REG as a way to tell
userspace that a given reg can't be changed after the vcpu started.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>