netif.h: describe request/response structures in terms of binary layout

netif.h: describe request/response structures in terms of binary layout

In RFC style, rather than relying on the implicit assumptions of a
particular C ABI.

I have also confirmed, using the Python gdb extension technique in
[0], that the struct offsets (in a Linux binary at least) are the same
as described here.

I took the opportunity to also confirm that x86_32, x86_64, arm32 and
arm64 are all the same.

This highlighted that struct netif_rx_request was missing some
explicit padding, which is added here.

Lastly, fixup some struct names to allow the generated docs to
properly hyperlink, mainly by adding the _t to type names where
appropriate, but also s/netif_tx_extra/netif_extra_info_t/.

[0] http://stackoverflow.com/questions/9788679/how-to-get-the-relative-adress-of-a-field-in-a-structure-dump-c

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>