tools/libxl: report trusted backend status to frontends

tools/libxl: report trusted backend status to frontends

Allow administrators to notify a frontend driver that it's backend
counterpart is not to be trusted, so the frontend can deploy whatever
mitigations required in order to secure itself.

Allow such option for disk and network frontends only, as those are
the only hardened ones currently supported.

This is part of XSA-403

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>