examples/vhost: support jumbo frame

examples/vhost: support jumbo frame

This patch support mergeable RX feature and thus support jumbo frame RX and TX
in user space vhost(as virtio backend).

On RX, it secures enough room from vring to accommodate one complete scattered
packet which is received by PMD from physical port, and then copy data from
mbuf to vring buffer, possibly across a few vring entries and descriptors.

On TX, it gets a jumbo frame, possibly described by a few vring descriptors which
are chained together with the flags of 'NEXT', and then copy them into one scattered
packet and TX it to physical port through PMD.

Signed-off-by: Changchun Ouyang <changchun.ouyang@intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>