libxl: write IO ABI for disk frontends

libxl: write IO ABI for disk frontends

This is a patch to forward-port a Xend behaviour. Xend writes IO ABI used for
all frontends. Blkfront before 2.6.26 relies on this behaviour otherwise guest
cannot boot when running in 32-on-64 mode. Blkfront after 2.6.26 writes that
node itself, in which case it's just an overwrite to an existing node which
should be OK.

In fact Xend writes the ABI for all frontends including console and vif. But
nowadays only old disk frontends rely on that behaviour so that we only write
the ABI for disk frontends in libxl, minimizing the impact.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>