autotest/autotest — pull request #549 by ypu (2012-09-18 12:01:15+00:00)
Title: virt: set vnet_hdr to True when open the tap device

Set vnet_hdr to True to let the open_tap to check if IFF_VNET_HDR
is support in tun. Otherwise most of the flags of the device will
be set to off and the performance of the nic device will be poor.

Signed-off-by: Yiqiao Pu