[EMULATORS] More sanity checks in callbacks provided by virtio_net and virtio_console

This patch reduces the probablity of crashing virtio_net and virtio_console emulators from guest.

In fact, all virtio emulators should take care of buggy or broken virtio programming from guest OS.

Signed-off-by: Anup Patel <anup@brainfault.org>
Merge remote-tracking branch 'upstream/master' into dev_exynos4