PCI: Can continually add funcs after adding func0

Boot up a KVM guest, and hotplug multifunction
devices(func1,func2,func0,func3) to guest.

for i in 1 2 0 3;do
qemu-img create /tmp/resize$i.qcow2 1G -f qcow2
(qemu) drive_add 0x11.$i id=drv11$i,if=none,file=/tmp/resize$i.qcow2
(qemu) device_add virtio-blk-pci,id=dev11$i,drive=drv11$i,addr=0x11.$i,multifunction=on
done

In linux kernel, when func0 of the slot is hot-added, the whole
slot will be marked as 'enabled', then driver will ignore other new
hotadded funcs.
But in Win7 & WinXP, we can continaully add other funcs after adding
func0, all funcs will be added in guest.

drivers/pci/hotplug/acpiphp_glue.c:
static int acpiphp_check_bridge(struct acpiphp_bridge *bridge)
{
....
        for (slot = bridge->slots; slot; slot = slot->next) {
                if (slot->flags & SLOT_ENABLED) {
                        acpiphp_disable_slot()
                else
                        acpiphp_enable_slot()
....                              |
}                                 v
                            enable_device()
                                  |
                                  v
        //only don't enable slot if func0 is not added
	list_for_each_entry(func, &slot->funcs, sibling) {
               ...
        }
       slot->flags |= SLOT_ENABLED; //mark slot to 'enabled'

This patch just make pci driver can continaully add funcs after adding
func 0. Only mark slot to 'enabled' when all funcs are added.

For pci multifunction hotplug, we can add functions one by one(func 0 is
necessary), and all functions will be removed in one time.

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>