apic: Don't iterate past last used apic

local_apics are allocated sequentially and never removed, so
we can stop any iterations that go to MAX_APICS as soon as we
hit the first NULL.  Looking at a small guest running a virtio-net
workload with oprofile, this drops apic_get_delivery_bitmask()
from #3 in the profile to down in the noise.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/apic.c

@@ -437,6 +437,8 @@ static int apic_find_dest(uint8_t dest)
         apic = local_apics[i];
 	if (apic && apic->id == dest)
             return i;
+        if (!apic)
+            break;
     }
 
     return -1;
@@ -472,6 +474,8 @@ static void apic_get_delivery_bitmask(uint32_t *deliver_bitmask,
                         set_bit(deliver_bitmask, i);
                     }
                 }
+            } else {
+                break;
             }
         }
     }