serial-pci: remove memory regions from BAR before destroying them

Otherwise, hot-unplug of pci-serial-2x trips the assertion
in memory_region_destroy:

    (qemu) device_del gg
    (qemu) qemu-system-x86_64: /work/armbru/tmp/qemu/memory.c:1021: memory_region_destroy: Assertion `((&mr->subregions)->tqh_first == ((void *)0))' failed.
    Aborted (core dumped)
Reported-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/char/serial-pci.c

@@ -148,6 +148,7 @@ static void multi_serial_pci_exit(PCIDevice *dev)
     for (i = 0; i < pci->ports; i++) {
         s = pci->state + i;
         serial_exit_core(s);
+        memory_region_del_subregion(&pci->iobar, &s->io);
         memory_region_destroy(&s->io);
         g_free(pci->name[i]);
     }