virtio: introduce PCI implementation structure

virtio: introduce PCI implementation structure

Introduce struct virtio_pci_ops, to let legacy virtio (v0.95) and
modern virtio (1.0) have different implementation regarding to a
specific pci action, such as read host status.

With that, this patch reimplements all exported pci functions, in
a way like:

	vtpci_foo_bar(struct virtio_hw *hw)
	{
		hw->vtpci_ops->foo_bar(hw);
	}

So that we need pay attention to those pci related functions only
while adding virtio 1.0 support.

This patch introduced a new vtpci function, vtpci_init(), to do
proper virtio pci settings. It's pretty simple so far: just sets
hw->vtpci_ops to legacy_ops as we don't support 1.0 yet.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: Qian Xu <qian.q.xu@intel.com>
Reviewed-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Tested-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Acked-by: Huawei Xie <huawei.xie@intel.com>