[Libguestfs] [PATCH nbdkit] xz: Do not pass can_write through to the plugin.

I'm not sure that this fix is really correct. An alternate way I can think to fix this would be for the core server to maintain a readonly flag for each layer (instead of just per- server). You could also argue that our readonly test in server/connections.c:compute_eflags is wrong and/or that the implementations of server/plugins.c:plugin_can_write and server/filters.c:filter_can_write have the wrong default - they should consult the readonly flag which was passed to that layer's open call. However the fix only affects one filter, and we're not maintaining an API for filters, so maybe we can defer the problem. Rich.