vhost: remove useless code for Rx/Tx

vhost: remove useless code for Rx/Tx

Remove all other codes and only keep virtio_dev_rx, copy_from_mbuf_to_vring,
virtio_dev_merge_rx, virtio_dev_merge_tx.

Previous vhost merge-able feature introduces another version of tx function,
virtio_dev_merge_tx. Actually it is not related to merge-able feature but is
the fix for memcpy between mbuf and vring descriptors.
This lib will create the tx functions based on virtio_dev_merge_tx.

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Acked-by: Changchun Ouyang <changchun.ouyang@intel.com>
[Thomas: do not remove code used or moved later]