Include new storage function volume_snapshot_list.

Returns a list of snapshots (youngest snap first) form a given volid.
It is possible to use a prefix to filter the list.

PVE/Storage/ZFSPlugin.pm

@@ -369,6 +369,12 @@ sub volume_has_feature {
     return undef;
 }
 
+sub volume_snapshot_list {
+    my ($class, $scfg, $storeid, $volname, $prefix, $ip) = @_;
+    # return an empty array if dataset does not exist.
+    die "Volume_snapshot_list is not implemented for ZFS over iSCSI.\n";
+}
+
 sub activate_storage {
     my ($class, $storeid, $scfg, $cache) = @_;