x86/mitigations: Fix the test for Xen PV guest

x86/mitigations: Fix the test for Xen PV guest

Commit 6af1c37c19ea ("x86/pti: Don't report XenPV as vulnerable")
looks at current hypervisor to determine whether we are running as a Xen
PV guest. This is incorrect since the test will be true for HVM guests
as well.

Instead we should see if we are xen_pv_domain().

(Using Xen-specific primitives in this file is not ideal. This is not
for upstream though so we are going to have to live with this)

Orabug: 29774291

Fixes: 6af1c37c19ea ("x86/pti: Don't report XenPV as vulnerable")
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Patrick Colp <patrick.colp@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>