Thomas Haller authored Apr 17, 2019 and Beniamino Galvani committed Apr 18, 2019

- if there is only one vlan in the list, then we can return success
  early. That is, because one NMBridgeVlan instance is always valid
  due to the way how users must use the API to construct the element.

- the implementation for check_normalizable is only correct, if there
  are no duplicate or overlapping ranges. Assert for that. In fact,
  all callers first check for errors and then for normalizable errors.

- avoid duplicate calls to nm_bridge_vlan_get_vid_range(). There are
  duplicate assertions that we don't need.

- only check for pvid once per range.

- combine calls to g_hash_table_contains() and g_hash_table_add().