x86: fix emuirq regression from XSA-21 fix

The XSA-21 patch broke the assumption of "ret" being zero prior to the
IRQ_UNBOUND check.

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

--- a/xen/arch/x86/physdev.c
+++ b/xen/arch/x86/physdev.c
@@ -243,6 +243,8 @@ static int physdev_unmap_pirq(struct phy
         spin_lock(&d->event_lock);
         if ( domain_pirq_to_emuirq(d, unmap->pirq) != IRQ_UNBOUND )
             ret = unmap_domain_pirq_emuirq(d, unmap->pirq);
+        else
+            ret = 0;
         spin_unlock(&d->event_lock);
         if ( unmap->domid == DOMID_SELF || ret )
             goto free_domain;