tools/libxl: env variable to signal whether disk/nic backend is trusted

tools/libxl: env variable to signal whether disk/nic backend is trusted

Introduce support in libxl for fetching the default backend trusted
option for disk and nic devices.

Users can set LIBXL_{DISK,NIC}_BACKEND_UNTRUSTED environment variable
to notify libxl of whether the backends for disk and nic devices
should be trusted.  Such information is passed into the frontend so it
can take the appropriate measures.

This is part of XSA-403.

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