qtest/libqos: add a function to initialize secondary PCI buses

Scan the PCI devices to find bridge and set PCI_SECONDARY_BUS and
PCI_SUBORDINATE_BUS (algorithm from seabios)

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211208130350.10178-2-lvivier@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>