RICHMOND, Va. (AP) _ Up to 300 members of the Virginia National Guard and the Virginia Defense Force will be staged in central Virginia and along the Interstate 81 corridor to help respond to a winter storm.   The National Guard says the soldiers, airmen and Virginia Defense Force members are expected to be in place at readiness centers by late Wednesday afternoon.   Winter storm warnings and watches are in effect across Virginia from Wednesday afternoon through Thursday night.   The National Weather Service says parts of central Virginia could receive 8 inches to a foot of snow.   Up to a foot of snow also is possible in northern, southwestern and Southside Virginia.   Gov Terry McAuliffe declared a state of emergency Tuesday ahead of the storm.