KVM: Clarify capability requirements for KVM_ARM_VCPU_FINALIZE

Userspace is only supposed to use KVM_ARM_VCPU_FINALIZE when there
is some vcpu feature that can actually be finalized.

This means that documenting KVM_ARM_VCPU_FINALIZE as available or
not depending on the capabilities present is not helpful.

This patch amends the documentation to describe availability in
terms of which capability is required for each finalizable feature
instead.

In any case, userspace sees the same error (EINVAL) regardless of
whether the given feature is not present or KVM_ARM_VCPU_FINALIZE
is not implemented at all.

No functional change.

Suggested-by: Andrew Jones <drjones@redhat.com>
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>