s390x/pci: Move some hotplug checks to the pre_plug handler

s390x/pci: Move some hotplug checks to the pre_plug handler

Let's move most of the checks to the new pre_plug handler. As a PCI
bridge is just a PCI device, we can simplify the code.

Notes: We cannot yet move the MSIX check or device ID creation +
zPCI device creation to the pre_plug handler as both parts are not
fixed before actual device realization (and therefore after pre_plug and
before plug). Once that part is factored out, we can move these parts to
the pre_plug handler, too and therefore remove all possible errors from
the plug handler.

Reviewed-by: Collin Walling <walling@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190114103110.10909-3-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>