qemu: remove redundant needReply argument of qemuAgentCommand

needReply added in [1] looks redundant. Indeed it is set to false only
when mon->await_event is set too (the only exception qemuAgentFSTrim
which is mistaken).

However it fixes the issue when qemuAgentCommand exits on error path and
mon->await_event is not reset. Let's instead reset mon->await_event properly.

Also remove "Woken up by event" debug message as it can be misleading.
We can get it also if monitor is closed due to serial changed event
currently. Anyway both qemuAgentClose and qemuAgentNotifyEvent log
itself.

[1] qemu: make sure agent returns error when required data are missing

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>