KVM: arm/arm64: Clean up vcpu finalization function parameter naming

Currently, the internal vcpu finalization functions use a different
name ("what") for the feature parameter than the name ("feature")
used in the documentation.

To avoid future confusion, this patch converts everything to use
the name "feature" consistently.

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>