rbd: priority to utilize 'at::PATH' rather than 'positional_path'

`rbd import` will return failed if only "--path" is specified
without "--dest-pool" and "--dest".

Signed-off-by: songweibin <song.weibin@zte.com.cn>

src/tools/rbd/Utils.cc

@@ -806,10 +806,10 @@ int get_image_size(const boost::program_options::variables_map &vm,
 
 int get_path(const boost::program_options::variables_map &vm,
              const std::string &positional_path, std::string *path) {
-  if (!positional_path.empty()) {
-    *path = positional_path;
-  } else if (vm.count(at::PATH)) {
+  if (vm.count(at::PATH)) {
     *path = vm[at::PATH].as<std::string>();
+  } else if (!positional_path.empty()) {
+    *path = positional_path;
   }
 
   if (path->empty()) {