mini-os: correct first free pfn

mini-os: correct first free pfn

The first free pfn available for allocation is calculated by adding the
number of page table frames to the pfn of the first page table and
then the magic number 3 to account for start info page et al.

As the start info page, xenstore page and console page are allocated
_before_ the page tables leaving room for these pages behind the page
tables makes no sense.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>