virt: move caching of virtualization check results into detect_vm() and detect_container()

After all, we ended up calling detect_container() more often than
detect_virtualization(), hence the former one should cache the results,
since the latter is only a wrapper around the former.

src/detect-virt/detect-virt.c

@@ -114,8 +114,8 @@ static int parse_argv(int argc, char *argv[]) {
 
 int main(int argc, char *argv[]) {
         const char *id = NULL;
-        int r;
         int retval = EXIT_SUCCESS;
+        int r;
 
         /* This is mostly intended to be used for scripts which want
          * to detect whether we are being run in a virtualized