PCI: Finish SR-IOV VF setup before adding the device

Commit 4f535093 "PCI: Put pci_dev in device tree as early as possible"
moves device registering from pci_bus_add_devices() to pci_device_add().
That causes problems for virtual functions because device_add(&virtfn->dev)
is called before setting the virtfn->is_virtfn flag, which then causes Xen
to report PCI virtual functions as PCI physical functions.

Fix it by setting virtfn->is_virtfn before calling pci_device_add().

[Jiang Liu]: Move the setting of virtfn->is_virtfn ahead further for better
readability and modify changelog.

Signed-off-by: Xudong Hao <xudong.hao@intel.com>
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org	# v3.9+