fix 2317: list images on 'rootdir' only storages

plugins can still override list_volumes if they want separate methods to
list rootdir and images content.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

PVE/Storage.pm

@@ -845,7 +845,7 @@ sub template_list {
 sub volume_list {
     my ($cfg, $storeid, $vmid, $content) = @_;
 
-    my @ctypes = qw(images vztmpl iso backup snippets);
+    my @ctypes = qw(rootdir images vztmpl iso backup snippets);
 
     my $cts = $content ? [ $content ] : [ @ctypes ];