domctl: restrict DOMCTL_set_target to HVM domains

Both the XSA-217 fix and
lists.xenproject.org/archives/html/xen-devel/2017-04/msg02945.html
make this assumption, so let's enforce it.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -1071,7 +1071,9 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xe
             break;
         }
 
-        ret = xsm_set_target(XSM_HOOK, d, e);
+        ret = -EOPNOTSUPP;
+        if ( is_hvm_domain(e) )
+            ret = xsm_set_target(XSM_HOOK, d, e);
         if ( ret ) {
             put_domain(e);
             break;