build: silence coverity false positives

Coverity complained about these intentional fallthrough cases, but
not about other cases that were explicitly marked with nice comments.

For some reason, Coverity doesn't seem smart enough to parse the
up-front English comment in virsh about intentional fallthrough :)

* tools/virsh.c (cmdVolSize): Mark fallthrough in a more typical
fashion.
* src/conf/nwfilter_conf.c (virNWFilterRuleDefDetailsFormat)
(virNWFilterRuleDetailsParse): Mark explicit fallthrough.

tools/virsh.c

@@ -7071,10 +7071,13 @@ static int cmdVolSize(const char *data, unsigned long long *val)
         switch (*end) {
         case 'T':
             *val *= 1024;
+            /* fallthrough */
         case 'G':
             *val *= 1024;
+            /* fallthrough */
         case 'M':
             *val *= 1024;
+            /* fallthrough */
         case 'k':
             *val *= 1024;
             break;