dev: Rename VirtIO PCI debug flag

Rename VIOPci -> VIOIface to avoid having a separate flag for the MMIO
interface.

Change-Id: I99f9210fa36ce33662c48537fd3992cd9a69d349
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Sudhanshu Jha <sudhanshu.jha@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Rekai Gonzalez Alberquilla <rekai.gonzalezalberquilla@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2324
Reviewed-by: Weiping Liao <weipingliao@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

dev: Add support for 9p proxying over VirtIO

This patch adds support for 9p filesystem proxying over VirtIO. It can
currently operate by connecting to a 9p server over a socket
(VirtIO9PSocket) or by starting the diod 9p server and connecting over
pipe (VirtIO9PDiod).


*WARNING*: Checkpoints are currently not supported for systems with 9p
proxies!

dev: Add a VirtIO block device model

dev: Add a VirtIO console device model

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.