x86, amd, xen: Avoid NULL pointer paravirt references

Stub out MSR methods that aren't actually needed.  This fixes a crash
as Xen Dom0 on AMD Trinity systems.  A bigger patch should be added to
remove the paravirt machinery completely for the methods which
apparently have no users!

Reported-by: Andre Przywara <andre.przywara@amd.com>
Link: http://lkml.kernel.org/r/20120530222356.GA28417@andromeda.dapyr.net


Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: <stable@vger.kernel.org>