x86: ioapic: fix fail migration when irqchip=split

Split irqchip works based on the fact that we kept the first 24 gsi
routing entries inside KVM for userspace ioapic's use. When system
boot, we'll reserve these MSI routing entries before hand. However,
after migration, we forgot to re-configure it up in the destination
side. The result is, we'll get invalid gsi routing entries after
migration (all empty), and we get interrupts with vector=0, then
strange things happen, like keyboard hang.

The solution is simple - we update them after migration, which is a
one line fix.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1483952153-7221-4-git-send-email-peterx@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
