Add a feature arc consistency check

Verify that last node in the computed feature order matches
reality. This check doesn't make sense in all cases, so we skip it if
the newly-added vnet_feature_arc_registration_t ".last_in_arc" datum
is a NULL pointer.

Change-Id: Ia99c3e2b2da2e4780a7d5bc71670c5742a66fef2
Signed-off-by: Dave Barach <dave@barachs.net>