xen/pciback: Use PCI device flag helper functions

Use PCI device flag helper functions when assigning or releasing device.
No functional change.

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>