vlog: Introduce VLOG_DEFINE_THIS_MODULE for declaring vlog module in use.

Adding a macro to define the vlog module in use adds a level of
indirection, which makes it easier to change how the vlog module must be
defined.  A followup commit needs to do that, so getting these widespread
changes out of the way first should make that commit easier to review.

lib/dpif-netdev.c

@@ -45,9 +45,9 @@
 #include "queue.h"
 #include "timeval.h"
 #include "util.h"
-
 #include "vlog.h"
-#define THIS_MODULE VLM_dpif_netdev
+
+VLOG_DEFINE_THIS_MODULE(dpif_netdev)
 
 /* Configuration parameters. */
 enum { N_QUEUES = 2 };          /* Number of queues for dpif_recv(). */