RISC-V: KVM: use ENOENT in *_one_reg() when extension is unavailable

Following a similar logic as the previous patch let's minimize the EINVAL
usage in *_one_reg() APIs by using ENOENT when an extension that
implements the reg is not available.

For consistency we're also replacing an EOPNOTSUPP instance that should
be an ENOENT since it's an "extension is not available" error.

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