vfio/common: Consolidate skip/invalid section into helper

vfio/common: Consolidate skip/invalid section into helper

The checks are replicated against region_add and region_del
and will be soon added in another memory listener dedicated
for dirty tracking.

Move these into a new helper for avoid duplication.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Avihai Horon <avihaih@nvidia.com>
Link: https://lore.kernel.org/r/20230307125450.62409-8-joao.m.martins@oracle.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>