tap: Remove qemu_using_vnet_hdr()

tap: Remove qemu_using_vnet_hdr()

Since qemu_set_vnet_hdr_len() is always called when
qemu_using_vnet_hdr() is called, we can merge them and save some code.

For consistency, express that the virtio-net header is not in use by
returning 0 with qemu_get_vnet_hdr_len() instead of having a dedicated
function, qemu_get_using_vnet_hdr().

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>