tools/virtio: propagate V=X to kernel build

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/Makefile

@@ -6,7 +6,7 @@ vringh_test: vringh_test.o vringh.o virtio_ring.o
 CFLAGS += -g -O2 -Werror -Wall -I. -I../include/ -I ../../usr/include/ -Wno-pointer-sign -fno-strict-overflow -fno-strict-aliasing -fno-common -MMD -U_FORTIFY_SOURCE
 vpath %.c ../../drivers/virtio ../../drivers/vhost
 mod:
-	${MAKE} -C `pwd`/../.. M=`pwd`/vhost_test
+	${MAKE} -C `pwd`/../.. M=`pwd`/vhost_test V=${V}
 .PHONY: all test mod clean
 clean:
 	${RM} *.o vringh_test virtio_test vhost_test/*.o vhost_test/.*.cmd \