qdev-monitor-test: Simplify using g_assert_cmpstr()

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>