xen/mce: Change the machine check point

xen/mce: Change the machine check point

This patch backport from Jeremy's pvops commit
073349667402682c997394b3fcdbbeb6707f368f

Enable MCE support in dom0, so that if a MCE happen and that MCE impact
dom0, dom0 can receive a vMCE.

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>
Signed-off-by: Ke, Liping <liping.ke@intel.com>
Signed-off-by: Jiang, Yunhong <yunhong.jiang@intel.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>