[PATCH/RFC] Fix xsave bug on older Xen hypervisors

[Stefan Bader]

When writing unsupported flags into CR4 (for some time the
xen_write_cr4 function would refuse to do anything at all)
older Xen hypervisors (and patch can potentially be improved
by finding out what older means in version numbers) would
crash the guest.

Since Amazon EC2 would at least in the past be affected by that,
Fedora and Ubuntu were carrying a hack that would filter out
X86_CR4_OSXSAVE before writing to CR4. This would affect any
PV guest, even those running on a newer HV.

And this recently caused trouble because some user-space was
only partially checking (or maybe only looking at the cpuid
bits) and then trying to use xsave even though the OS support
was not set.

So I came up with a patch that would
- limit the work-around to certain Xen versions
- prevent the write to CR4 by unsetting xsave and osxsave in
  the cpuid bits

Doing things that way may actually allow this to be acceptable
upstream, so I am sending it around, now.
It probably could be improved when knowing the exact version
to test for but otherwise should allow to work around the guest
crash while not preventing xsave on Xen 4.x and newer hosts.

-Stefan