vhost: improve performance by supporting large buffer

vhost: improve performance by supporting large buffer

The rte_vhost_dequeue_burst supports two ways of dequeuing data.
If the data fits into a buffer, then all data is copied and a
single linear buffer is returned. Otherwise it allocates
additional mbufs and chains them together to return a multiple
segments mbuf.

While that covers most use cases, it forces applications that
need to work with larger data sizes to support multiple segments
mbufs. The non-linear characteristic brings complexity and
performance implications to the application.

To resolve the issue, add support to attach external buffer
to a pktmbuf and let the host provide during registration if
attaching an external buffer to pktmbuf is supported and if
only linear buffer are supported.

Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>