net: move net-checksum.c under net/

Also add a new net/checksum.h header

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 4acdd12..ac919e1 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -13,6 +13,7 @@
 
 #include "virtio.h"
 #include "net.h"
+#include "net/checksum.h"
 #include "qemu-timer.h"
 #include "virtio-net.h"