x86: vMCE save and restore

x86: vMCE save and restore

This patch provides vMCE save/restore when migration.
1. MCG_CAP is well-defined. However, considering future cap extension,
   we keep save/restore logic that Jan implement at c/s 24887;
2. MCi_CTL2 initialized by guestos when booting, so need save/restore
   otherwise guest would surprise;
3. Other MSRs do not need save/restore since they are either error-
   related and pointless to save/restore, or, unified among all vMCE
   platform;

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>
- fix handling of partial data in XEN_DOMCTL_set_ext_vcpucontext
- fix adjustment of xen_domctl_ext_vcpucontext

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Committed-by: Jan Beulich <jbeulich@suse.com>