xen/mce: Add mcelog support for Xen platform

xen/mce: Add mcelog support for Xen platform

When MCA error occurs, it would be handled by Xen hypervisor first,
and then the error information would be sent to initial domain for logging.

This patch gets error information from Xen hypervisor and convert
Xen format error into Linux format mcelog. This logic is basically
self-contained, not  touching other kernel components.

By using tools like mcelog tool users could read specific error information,
like what they did under native Linux.

To test follow directions outlined in Documentation/acpi/apei/einj.txt

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>
Conflicts:

	drivers/xen/Kconfig
	drivers/xen/Makefile