KVM: VMX: Use wrmsr for switching between guest and host IA32_XSS on Intel

When the guest can execute the XSAVES/XRSTORS instructions, use wrmsr to
set the hardware IA32_XSS MSR to guest/host values on VM-entry/VM-exit,
rather than the MSR-load areas. By using the same approach as AMD, we
will be able to use a common implementation for both (in the next
patch).

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Change-Id: I9447d104b2615c04e39e4af0c911e1e7309bf464
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>