Fix save/restore when using grant tables in network, robust-ify netback

Fix save/restore when using grant tables in network, robust-ify netback
against bogus front-end drivers, plus various small cleanups. Note that
there is now only /one/ config option controlling the use of grant tables
with networking (not separate ones for rx and tx).

An outstanding issue is that console reconnect after a restore causes badness.

Signed-off-by: Steven Hand <steven@xensource.com>