x86: don't change affinity with interrupt unmasked

With ->startup unmasking the IRQ, setting the affinity afterwards
without masking the IRQ again is invalid namely for MSI (which can't
have their affinity updated atomically).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Changing the affinity of non-maskable MSI IRQs seems bogus too, but I
can't immediately see what we can do about this (better than disabling
affinity changes for them).

--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -1608,12 +1608,13 @@ int pirq_guest_bind(struct vcpu *v, stru
         init_timer(&action->eoi_timer, irq_guest_eoi_timer_fn, desc, 0);
 
         desc->status |= IRQ_GUEST;
-        desc->status &= ~IRQ_DISABLED;
-        desc->handler->startup(desc);
 
         /* Attempt to bind the interrupt target to the correct CPU. */
         if ( !opt_noirqbalance && (desc->handler->set_affinity != NULL) )
             desc->handler->set_affinity(desc, cpumask_of(v->processor));
+
+        desc->status &= ~IRQ_DISABLED;
+        desc->handler->startup(desc);
     }
     else if ( !will_share || !action->shareable )
     {