xen: Use dev_is_pci() to check whether it is pci device

Use PCI standard marco dev_is_pci() instead of directly compare
pci_bus_type to check whether it is pci device.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jan Beulich <JBeulich@suse.com>