Page offline support in Xen side

Page offline support in Xen side

This patch add support to offline a page. The basical idea is, when a
page is assigned, it will be marked offline pending and be moved out of
buddy when freed, when a page is free, it will be moved out of buddy directly.

One notice after this change is, now the page->count_info is not
always 0, especially for shadow page, since the PGC_offlining bit may be set.

Signed-off-by: Wang, Shane <shane.wang@intel.com>
Signed-off-by: Jiang, Yunhong <yunhong.jiang@intel.com>