hw/pcie: better hotplug/hotunplug support

The current code is broken: it does surprise removal which crashes guests.

Reimplemented the steps:
 - Hotplug triggers both 'present detect change' and
   'attention button pressed'.

 - Hotunplug starts by triggering 'attention button pressed',
   then waits for the OS to power off the device and only
   then detaches it.

Fixes CVE-2014-3471.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
