map_volume: fall back to 'path'

Adds a fallback to 'Plugin::path' in the default implementation of
'map_volume' to simplify a common case of calling 'map_volume' followed
by a defined-check and a call to path if it is not. The path is now
always returned if the plugin in question does not override
'map_volume'.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>

PVE/Storage/Plugin.pm

@@ -966,7 +966,8 @@ sub deactivate_storage {
 sub map_volume {
     my ($class, $storeid, $scfg, $volname, $snapname) = @_;
 
-    return undef;
+    my ($path) = $class->path($scfg, $volname, $storeid, $snapname);
+    return $path;
 }
 
 sub unmap_volume {