dev, pci: Implement basic VirtIO support

This patch adds support for VirtIO over the PCI bus. It does so by
providing the following new SimObjects:

* VirtIODeviceBase - Abstract base class for VirtIO devices.
* PciVirtIO - VirtIO PCI transport interface.

A VirtIO device is hooked up to the guest system by adding a PciVirtIO
device to the PCI bus and connecting it to a VirtIO device using the
vio parameter.

New VirtIO devices should inherit from VirtIODevice base and
implementing one or more VirtQueues. The VirtQueues are usually
device-specific and all derive from the VirtQueue class. Queues must
be registered with the base class from the constructor since the
device assumes that the number of queues stay constant.