hw/9pfs: add xattr count query interface to fs synth driver Add a synthetic "/stat/xattr_count" file path that, if being read by 9p client, returns the 9p server internal xattr FID counter to client. This allows to test and verify that the xattr FID limit is being enforced correctly. Link: https://lore.kernel.org/qemu-devel/357c20fc244c04dcbd36b13aa20a5c9b2034e9f0.1781361555.git.qemu_oss@crudebyte.com Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>