x86/domctl: generalize the restore of vMCE parameters

x86/domctl: generalize the restore of vMCE parameters

vMCE parameters in struct xen_domctl_ext_vcpucontext were extended in
the past, and is likely to be extended in the future. When migrating a
PV domain from old Xen, XEN_DOMCTL_set_ext_vcpucontext should handle
the differences.

Instead of adding ad-hoc handling code at each extension, we introduce
an array to record sizes of the current and all past versions of vMCE
parameters, and search for the largest one that does not expire the
size of passed-in parameters to determine vMCE parameters that will be
restored. If vMCE parameters are extended in the future, we only need
to adapt the array to reflect the extension.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>