xen/balloon: set a mapping for ballooned out pages

Currently ballooned out pages are mapped to 0 and have INVALID_P2M_ENTRY
in the p2m. These ballooned out pages are used to map foreign grants
by gntdev and blkback (see alloc_xenballooned_pages).

Allocate a page per cpu and map all the ballooned out pages to the
corresponding mfn. Set the p2m accordingly. This way reading from a
ballooned out page won't cause a kernel crash (see
http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html

).

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
CC: alex@alex.org.uk
CC: dcrisan@flexiant.com
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>