fix: error out if snapname is set in filesystem_path

Pure currently does not support direct mounting of snapshots.
As we currently silently ignore the snapname field at least error
out if we receive a non empty snapname. Its unclear if this is
still used as vzdump nowadays does live backups.

Signed-off-by: Peter Lieven <pl@dlhnet.de>

PureStoragePlugin.pm

@@ -892,6 +892,8 @@ sub filesystem_path {
   my ( $class, $scfg, $volname, $snapname ) = @_;
   print "Debug :: PVE::Storage::Custom::PureStoragePlugin::sub::filesystem_path\n" if $DEBUG;
 
+  die "Error :: filesystem_path: snapshot is not implemented ($snapname)\n" if defined($snapname);
+
   # do we even need this?
   my ( $vtype, undef, $vmid ) = $class->parse_volname( $volname );