add subvol support for directory storage

This storage type does not support subvol quotas, so we
only allow this if vdisk_alloc is called with size=0.

PVE/Storage/DirPlugin.pm

@@ -18,7 +18,7 @@ sub plugindata {
     return {
 	content => [ { images => 1, rootdir => 1, vztmpl => 1, iso => 1, backup => 1, none => 1 },
 		     { images => 1,  rootdir => 1 }],
-	format => [ { raw => 1, qcow2 => 1, vmdk => 1 } , 'raw' ],
+	format => [ { raw => 1, qcow2 => 1, vmdk => 1, subvol => 1 } , 'raw' ],
     };
 }