xen/p2m: Move code around to allow for better re-usage.

We are going to be using the early_alloc_p2m (and
early_alloc_p2m_middle) code in follow up patches which
are not related to setting identity pages.

Hence lets move the code out in its own function and
rename them as appropiate.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>