RISC-V: KVM: return ENOENT in *_one_reg() when reg is unknown

get_one_reg() and set_one_reg() are returning EINVAL errors for almost
everything: if a reg doesn't exist, if a reg ID is malformatted, if the
associated CPU extension that implements the reg isn't present in the
host, and for set_one_reg() if the value being written is invalid.

This isn't wrong according to the existing KVM API docs (EINVAL can be
used when there's no such register) but adding more ENOENT instances
will make easier for userspace to understand what went wrong.

Existing userspaces can be affected by this error code change. We
checked a few. As of current upstream code, crosvm doesn't check for any
particular errno code when using kvm_(get|set)_one_reg(). Neither does
QEMU. rust-vmm doesn't have kvm-riscv support yet. Thus we have a good
chance of changing these error codes now while the KVM RISC-V ecosystem
is still new, minimizing user impact.

Change all get_one_reg() and set_one_reg() implementations to return
-ENOENT at all "no such register" cases.

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