ACPI: don't call acpi_pcihp_device_plug_cb on xen

Commit f0c9d64a exposed the issue that with a xenfv machine using
pci passthrough, acpi pci hotplug code was being executed by mistake.
Guard calls to acpi_pcihp_device_plug_cb (and corresponding
acpi_pcihp_device_unplug_cb) with a check for xen_enabled(). Without
this check I am seeing an error that the bus doesn't have the
acpi-pcihp-bsel property set.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
