PCI, xen-pcifront: use new PCI interfaces to simplify implementation

Use new PCI interfaces to simplify xen-pcifront implementation:
1) Use pci_create_root_bus() instead of pci_scan_bus_parented()
   because pci_scan_bus_parented() is marked as __deprecated.This
   also gets rid of a duplicated call of pci_bus_start_devices().
2) Use pci_stop_root_bus() and pci_remove_root_bus() instead of
   open-coded private implementation.
3) Use pci_set_host_bridge_release() to release data structures
   associated with PCI root buses.
4) Use pci_bus_get()/pci_bus_put() to manage PCI root bus reference
   count.

This is also a preparation for coming PCI bus lock enhancement.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: xen-devel@lists.xensource.com
Cc: virtualization@lists.linux-foundation.org
Cc: linux-pci@vger.kernel.org
Cc: linux-kernel@vger.kernel.org