Describe the virtio device so we can mount disk images in the simulator.

[Reduced the size of the region based on feedback from review -- broonie]
Signed-off-by: NMark Hambleton <mahamble@broadcom.com>
Signed-off-by: NMark Brown <broonie@linaro.org>
Acked-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>