x86: PHYSDEVOP_{prepare,release}_msix are privileged

Yet this wasn't being enforced.

This is XSA-87.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/physdev.c
+++ b/xen/arch/x86/physdev.c
@@ -554,7 +554,9 @@ ret_t do_physdev_op(int cmd, XEN_GUEST_H
     case PHYSDEVOP_release_msix: {
         struct physdev_pci_device dev;
 
-        if ( copy_from_guest(&dev, arg, 1) )
+        if ( !IS_PRIV(v->domain) )
+            ret = -EPERM;
+        else if ( copy_from_guest(&dev, arg, 1) )
             ret = -EFAULT;
         else if ( dev.seg )
             ret = -EOPNOTSUPP;