BugLink: http://bugs.launchpad.net/bugs/1607374

Mounting proc in user namespace containers fails if the xenbus
filesystem is mounted on /proc/xen because this directory fails
the "permanently empty" test. proc_create_mount_point() exists
specifically to create such mountpoints in proc but is currently
proc-internal. Export this interface to modules, then use it in
xenbus when creating /proc/xen.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>