xen/9pfs: increase max ring order to 9

The max order allowed by the protocol is 9. Increase the max order
supported by QEMU to 9 to increase performance.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <20200521192627.15259-3-sstabellini@kernel.org>
Signed-off-by: Greg Kurz <groug@kaod.org>