Virginia’s top housing official cautioned yesterday that serious danger is on the horizon for tens of thousands in the region whose home mortgages “are premised on escalating home prices.” William Shelton, director of the Virginia Department of Housing and Community Development, told an audience gathered for the annual awards breakfast of the A-Home affordable housing advocacy group in Tysons Corner that “if there is a decline in home prices, there are going to be a lot of folks in a very difficult situation.”