block: handle ENOTSUP from discard in generic code

Similar to write_zeroes, let the generic code receive a ENOTSUP for
discard operations.  Since bdrv_discard has advisory semantics,
we can just swallow the error.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
