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>
