x86 hvm: Do not overwrite boot-cpu capability data on VMX/SVM startup.

x86 hvm: Do not overwrite boot-cpu capability data on VMX/SVM startup.

Apparently required back in the earliest days of Xen, we now properly
initialise CPU capabilities early during bootstrap. Re-writing
capability data later now causes problems if specific features have
been deliberately masked out.

Thanks to Weidong Han at Intel for finding such a bug where XSAVE
feature is masked out by default, but then erroneously written back
during VMX initialisation. This would cause memory corruption problems
during boot for XSAVE-capable systems.

Signed-off-by: Keir Fraser <keir@xen.org>