Force x86_xsave_features to 0 when running under XEN for AMD
processors.  This prevents the use of xsave and xrstor thus fixing
the problem in PR/49150.  The basic problem is that the way AMD
implements those instructions means that information can leak
between domains so XEN treats them as privileged.

XXX If anybody else comes up with a better / more "proper" fix, go
for it.  However, this solves the problem I was having.  And, given
that XEN being broken is pretty much a show-stopper for a release,
something needed to be done.