qemurunner: avoid leaking server_socket

close() the server_socket before qemusock replaces
it.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit be7bbbee)
Signed-off-by: Steve Sakoman <steve@sakoman.com>