[EMULATORS] First-cut implementation of VirtIO framework and VirtIO MMIO transport device

Signed-off-by: Anup Patel <anup@brainfault.org>
[EMULATORS] net: Add experimental VirtIO network device emulator

Signed-off-by: Anup Patel <anup@brainfault.org>
[TESTS] Enable basic VirtIO drivers for linux-3.7.4 on Realview-PB-A8 guest

Signed-off-by: Anup Patel <anup@brainfault.org>