Any county officer who resides in the county or in any town therein, and has an established home therein, which homesite becomes a part of a city after such officer’s election or appointment, shall not vacate his office by reason of his residence in the city, but shall continue to hold such office so long as he is successively elected or appointed to the office held by him at the time of the transition. Such officer shall for the purpose of his office be deemed to be a resident of the magisterial district in which the homesite was before becoming a part of a city. This section shall not apply to members of the school board of such county, who shall be governed by § 22.1-29.