xen/9pfs: introduce Xen 9pfs transport driver

xen/9pfs: introduce Xen 9pfs transport driver

Introduce the Xen 9pfs transport driver: add struct xenbus_driver to
register as a xenbus driver and add struct p9_trans_module to register
as v9fs driver.

All functions are empty stubs for now.

CC: [email protected]
CC: [email protected]
CC: Eric Van Hensbergen <[email protected]>
CC: Ron Minnich <[email protected]>
CC: Latchesar Ionkov <[email protected]>
CC: [email protected]
Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>