examples/vhost: fix vlan offload
examples/vhost: fix vlan offload

The following commit break vm2vm hard mode test cases:
commit db4014f2b65cb31bf ("use factorized default Rx/Tx configuration")

Investigation show that it needs enabling vlan offload since it is turn off
by default in some drivers, and Tx need it, especially when vm2vm is in hard mode.

Signed-off-by: Changchun Ouyang <593bf1f161e79772915ea885e47cf0e95f4498c0@intel.com>
Tested-by: Jingguo Fu <8ceb7d16df4fa08bdeade33f2c2a403a140310c2@intel.com>

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