tests/device-plug: Add a simple PCI unplug request test

tests/device-plug: Add a simple PCI unplug request test

The issue with testing asynchronous unplug requests it that they usually
require a running guest to handle the request. However, to test if
unplug of PCI devices works, we can apply a nice little trick on some
architectures:

On system reset, x86 ACPI, s390x and spapr will perform the unplug,
resulting in the device of interest to get deleted and a DEVICE_DELETED
event getting sent.

On s390x, we still get a warning
    qemu-system-s390x: -device virtio-mouse-pci,id=dev0:
    warning: Plugging a PCI/zPCI device without the 'zpci' CPU feature
    enabled; the guest will not be able to see/use this device

This will be fixed soon, when we enable the zpci CPU feature always
(Conny already has a patch for this queued).

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190218092202.26683-4-david@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>