[EMULATORS] virtio_queue: Simplify virtio_queue_get_iovec() and introduce virtio_queue_get_head_iovec()

The upcoming virtio block emulator requires virtio_queue_get_head_iovec() API.

Also, virtio_queue_get_head_iovec() is more generic hence virtio_queue_get_iovec() is implemented using virtio_queue_get_head_iovec()

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