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/coverage.c

@@ -23,10 +23,10 @@
 #include "hash.h"
 #include "unixctl.h"
 #include "util.h"
-
-#define THIS_MODULE VLM_coverage
 #include "vlog.h"
 
+VLOG_DEFINE_THIS_MODULE(coverage)
+
 static unsigned int epoch;
 
 static void