Revert "ACPI: don't call acpi_pcihp_device_plug_cb on xen"

This reverts commit 153eba4726dfa1bdfc31d1fe973b2a61b9035492.

This patch prevents PCI passthrough hotplug on Xen. Even if the Xen tool
stack prepares its own ACPI tables, we still rely on QEMU for hotplug
ACPI notifications.

The original issue is fixed by the two previous patch:
  hw/acpi: Limit hotplug to root bus on legacy mode
  hw/acpi: Move acpi_set_pci_info to pcihp

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
