libxl: create a local xenstore libxl and device-model dir for guests

libxl: create a local xenstore libxl and device-model dir for guests

If libxl is executed inside a guest domain it needs write access to
the local libxl xenstore dir (/local/<domid>/libxl) to store internal
data. This also applies to Qemu which needs a
/local/<domid>/device-model xenstore directory.

This patch creates the mentioned directories for each guest launched
from libxl.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>