xen: correctly rebuild mfn list list after migration.

Otherwise the second migration attempt fails because the mfn_list_list
still refers to all the old mfns.

We need to update the entires in both p2m_top_mfn and the mid_mfn
pages which p2m_top_mfn refers to.

In order to do this we need to keep track of the virtual addresses
mapping the p2m_mid_mfn pages since we cannot rely on
mfn_to_virt(p2m_top_mfn[idx]) since p2m_top_mfn[idx] will still
contain the old MFN after a migration, which may now belong to another
domain and hence have a different mapping in the m2p.

Therefore add and maintain a third top level page, p2m_top_mfn_p[],
which tracks the virtual addresses of the mfns contained in
p2m_top_mfn[].

We also need to update the content of the p2m_mid_missing_mfn page on
resume to refer to the page's new mfn.

p2m_missing does not need updating since the migration process takes
care of the leaf p2m pages for us.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>