qemuhotplugtest: Fix misleading comment on monitor unlock

qemuhotplugtest: Fix misleading comment on monitor unlock

There's a comment in testQemuHotplug() trying to explain why we
need to unlock the monitor object. Well, while it might have been
correct when being introduced, it's no longer factually correct
as just any function (attach/detach/update) might talk to the
monitor and it expects the monitor to be unlocked (as it calls
qemuDomainObjEnterMonitor() + qemuDomainObjExitMonitor()).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>