vm-virtio: Add PCI transport support

Copied from crosvm 107edb3e with one main modification: VirtioPciDevice
implements BusDevice.

We need this modification because it is the only way for us to be able
to add a VirtioPciDevice to the MMIO bus. Bus insertion takes a
BusDevice. The fact that VirtioPciDevice implements PciDevice which
itself implements BusDevice does not mean that Rust will automatically
downcast a VirtioPciDevice into a BusDevice.

crosvm works around that issue by having the PCI, virtio and BusDevice
implementations in the same crate.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

vm-virtio/src/transport/mod.rs

@@ -0,0 +1,7 @@
+// Copyright (c) 2019 Intel
+
+mod pci_common_config;
+mod pci_device;
+
+pub use pci_common_config::VirtioPciCommonConfig;
+pub use pci_device::VirtioPciDevice;