xen p2m: transparently change the p2m mappings in the m2p override
xen p2m: transparently change the p2m mappings in the m2p override

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: Stefano Stabellini <22a0c6385d750d07aa383c9f813cf6f0228657d8@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <da3a51b335cef0eb0e2c329c5ef6bcd6acef687a@oracle.com>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas