tests/acpi: add new unit test to test hotplug off/on feature on the root pci bus

tests/acpi: add new unit test to test hotplug off/on feature on the root pci bus

Ability to turn hotplug off on the pci root bus for i440fx was added in commit:
3d7e78aa7777f0 ("Introduce a new flag for i440fx to disable PCI hotplug on the
root bus")

This change adds a unit test in order to test this feature.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200918084111.15339-3-ani@anisinha.ca>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>