passthrough: support the assignment of the VF of Intel 82599 10GbE Controller

passthrough: support the assignment of the VF of Intel 82599 10GbE Controller

The datasheet is available at
http://download.intel.com/design/network/datashts/82599_datasheet.pdf

See 'Table 9.7. VF PCIe Configuration Space' of the datasheet, the PCI
Express Capability Structure of the VF of Intel 82599 10GbE Controller looks
trivial, e.g., the PCI Express Capabilities Register is 0, so the Capability
Version is 0 and pt_pcie_size_init() would fail.

We should not try to expose the PCIe cap of the device to guest.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>