fix #2226: allow broken symlinks to be deleted via API

Symlinks with a non-existing target fail Perls '-f' test and were thus
not deleteable via the API (failing with '$path does not exist').

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

PVE/Storage/Plugin.pm

@@ -689,8 +689,8 @@ sub free_image {
     if (defined($format) && ($format eq 'subvol')) {
 	File::Path::remove_tree($path);
     } else {
-    
-	if (! -f $path) {
+
+	if (! (-f $path || -l $path)) {
 	    warn "disk image '$path' does not exists\n";
 	    return undef;
 	}