x86/vioapic: clear remote IRR when switching RTE to edge triggered mode

Xen itself (as much as Linux) relies on this behavior, so it should
also emulate it properly. Not doing so reportedly gets in the way of
kexec inside a HVM guest.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Olaf Hering <olaf@xxxxxxxxx>

--- a/xen/arch/x86/hvm/vioapic.c
+++ b/xen/arch/x86/hvm/vioapic.c
@@ -154,8 +154,9 @@ static void vioapic_write_redirent(
     {
         vlapic_adjust_i8259_target(d);
     }
-    else if ( (ent.fields.trig_mode == VIOAPIC_LEVEL_TRIG) &&
-              !ent.fields.mask &&
+    else if ( ent.fields.trig_mode == VIOAPIC_EDGE_TRIG )
+        pent->fields.remote_irr = 0;
+    else if ( !ent.fields.mask &&
               !ent.fields.remote_irr &&
               hvm_irq->gsi_assert_count[idx] )
     {