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: NAndrew Jones <drjones@redhat.com>
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Reviewed-by: NAndrew Jones <drjones@redhat.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>