vhost: calculate mbuf size
vhost: calculate mbuf size

As a lib, we have no idea the app defined mbuf size.
This patch will calculate mbuf size dynamically.

Signed-off-by: Huawei Xie <3b3be4428106761300a2ee559c9f71ecdb091553@intel.com>
Acked-by: Changchun Ouyang <593bf1f161e79772915ea885e47cf0e95f4498c0@intel.com>

msune/dpdk,venkynv/dpdk-mirror,mixja/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,msune/dpdk,msune/dpdk,venkynv/dpdk-mirror,mixja/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork,venkynv/dpdk-mirror,mixja/dpdk,john-mcnamara-intel/dpdk