dir plugin: get notes: return undef if notes are not supported

This avoids showing empty notes in the result of the content/{volid}
API call for volumes that do not even support notes. It's also in
preparation for the proposed get_volume_attribute generalization,
which expects undef to be returned when an attribute is not supported.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>

PVE/Storage/DirPlugin.pm

@@ -94,6 +94,9 @@ sub parse_is_mountpoint {
 sub get_volume_notes {
     my ($class, $scfg, $storeid, $volname, $timeout) = @_;
 
+    my ($vtype) = $class->parse_volname($volname);
+    return if $vtype ne 'backup';
+
     my $path = $class->filesystem_path($scfg, $volname);
     $path .= $class->SUPER::NOTES_EXT;