RISC-V: KVM: use EBUSY when !vcpu->arch.ran_atleast_once

vcpu_set_reg_config() and vcpu_set_reg_isa_ext() is throwing an
EOPNOTSUPP error when !vcpu->arch.ran_atleast_once. In similar cases
we're throwing an EBUSY error, like in mvendorid/marchid/mimpid
set_reg().

EOPNOTSUPP has a conotation of finality. EBUSY is more adequate in this
case since its a condition/error related to the vcpu lifecycle.

Change these EOPNOTSUPP instances to EBUSY.

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