x86: flush high xstate CPUID sub-leaves to zero

x86: flush high xstate CPUID sub-leaves to zero

In line with other recent changes, these should be fully white listed,
requiring us to zero them until they obtain a meaning we support.

Without XSAVE support, all xstate sub-leaves should be zero.

Also move away from checking host XSAVE support - we really ought to
consider the guest flag for that purpose.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>