In m2p_add_override store the original mfn into page->index and then
change the p2m mapping, setting mfns as FOREIGN_FRAME.

In m2p_remove_override restore the original mapping.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>