x86/hvm: load CPU structures from xen versions <=3.4

x86/hvm: load CPU structures from xen versions <=3.4

Xen 4.0 added "msr_tsc_aux" in the middle of the hvm_hw_cpu structure,
making it incompatible with pre-3.4 savefiles.  This patch uses the
recently introduced backwards-compatibility infrastructure to convert
the old to the new.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Tim Deegan <Tim.Deegan@citrix.com>
Committed-by: Tim Deegan <Tim.Deegan@citrix.com>
xen-unstable changeset:   23172:dc8b05d22a59
xen-unstable date:        Wed Apr 06 11:40:54 2011 +0100