hw/xen: populate store frontend nodes with XenStore PFN/port

This is kind of redundant since without being able to get these through
some other method (HVMOP_get_param) the guest wouldn't be able to access
XenStore in order to find them.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>