minios: don't rely on specific page table allocation scheme

minios: don't rely on specific page table allocation scheme

Today mini-os is making assumptions how the page tables it is started
with are being allocated. Especially it is using the number of page
table frames to calculate which is the first unmapped pfn.

Instead of relying on page table number assumptions just look into the
page tables to find the first pfn not already mapped.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>