vhost: refactor mergeable Rx

vhost: refactor mergeable Rx

Current virtio_dev_merge_rx() implementation just looks like the
old rte_vhost_dequeue_burst(), full of twisted logic, that you
can see same code block in quite many different places.

However, the logic of virtio_dev_merge_rx() is quite similar to
virtio_dev_rx().  The big difference is that the mergeable one
could allocate more than one available entries to hold the data.
Fetching all available entries to vec_buf at once makes the
difference a bit bigger then.

The refactored code looks like below:

	while (mbuf_has_not_drained_totally || mbuf_has_next) {
		if (this_desc_has_no_room) {
			this_desc = fetch_next_from_vec_buf();

			if (it is the last of a desc chain)
				update_used_ring();
		}

		if (this_mbuf_has_drained_totally)
			mbuf = fetch_next_mbuf();

		COPY(this_desc, this_mbuf);
	}

This patch reduces quite many lines of code, therefore, make it much
more readable.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>