qemuhotplugtest: detach: Remove commands which are not issued

The 'human-monitor-command' equates to the 'drive-del' command issued by
the hotplug code on successful detach of a device. This means that it's
not issued for failed attempts and thus should not be added to the
expected list. Unfortunately our test monitor doesn't ensure that all
expected commands were consumed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>