When failing to restore the ITT for a DTE, we should remove the failed
device entry from the list and free the object.

We slightly refactor vgic_its_destroy to be able to reuse the now
separate vgic_its_free_dte() function.
Signed-off-by: NChristoffer Dall <cdall@linaro.org>
Reviewed-by: NEric Auger <eric.auger@redhat.com>