KVM: Clarify KVM_{SET,GET}_ONE_REG error code documentation

The current error code documentation for KVM_GET_ONE_REG and
KVM_SET_ONE_REG could be read as implying that all architectures
implement these error codes, or that KVM guarantees which error
code is returned in a particular situation.

Because this is not really the case, this patch waters down the
documentation explicitly to remove such guarantees.

EPERM is marked as arm64-specific, since for now arm64 really is
the only architecture that yields this error code for the
finalization-required case.  Keeping this as a distinct error code
is useful however for debugging due to the statefulness of the API
in this instance.

No functional change.

Suggested-by: Andrew Jones <drjones@redhat.com>
Fixes: 395f562f ("KVM: Document errors for KVM_GET_ONE_REG and KVM_SET_ONE_REG")
Fixes: 50036ad0 ("KVM: arm64/sve: Document KVM API extensions for SVE")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>