qdev-monitor-test: Simplify using g_assert_cmpstr()

Use g_assert_cmpstr() instead of combining g_assert() and strcmp(3).
This simplifies the code since we no longer have to play games to
distinguish NULL from "" using "(null)".

gcc extension haters will also be happy that ?: was dropped.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
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 <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>