xen: implement unplug protocol in xen_platform

xen: implement unplug protocol in xen_platform

The unplug protocol is necessary to support PV drivers in the guest: the
drivers expect to be able to "unplug" emulated disks and nics before
initializing the Xen PV interfaces.
It is responsibility of the guest to make sure that the unplug is done
before the emulated devices or the PV interface start to be used.

We use pci_for_each_device to walk the PCI bus, identify the devices and
disks that we want to disable and dynamically unplug them.

Changes in v2:

- use PCI_CLASS constants;

- replace pci_unplug_device with qdev_unplug;

- do not import hw/ide/internal.h in xen_platform.c;

Changes in v3:

- introduce piix3-ide-xen, that support hot-unplug;

- move the unplug code to hw/ide/piix.c;

- just call qdev_unplug from xen_platform.c to unplug the IDE disks;

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>