hw/acpi: Limit hotplug to root bus on legacy mode

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>
(cherry picked from commit f5855994)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>