net: fix TAP networking on host kernels without IFF_VNET_HDR support

vnet_hdr is initialized at 1 by default. We need to reset it to 0 if
the kernel doesn't support IFF_VNET_HDR.

Signed-off-by: Pierre Riteau <Pierre.Riteau@irisa.fr>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
