net: initialize vnet_hdr in net_init_tap()

net_tap_init() always sets vnet_hdr using qemu_opt_get_bool(), but
initialize it in net_init_tap() just to reduce confusion.

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