qdev-monitor-test: Don't test human-readable error message

qdev-monitor-test: Don't test human-readable error message

Test the error class instead.  Expecting a specific message is
fragile.  In fact, it broke once already, in commit 75884af.  Restore
the test of error member "class" dropped there, and drop the test of
error member "desc".

There are no other tests of "desc" as far as I can tell.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>