xen/pcifront: Don't use deprecated function pci_scan_bus_parented()

Use pci_scan_root_bus() instead of deprecated function
pci_scan_bus_parented().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: xen-devel@lists.xenproject.org