monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB

monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB

The QMP dispatcher coroutine holds the qmp_queue_lock over a yield
point, where it expects to be rescheduled from the main context. If a
CHR_EVENT_CLOSED event is received just then, it can race and block the
main thread on the mutex in monitor_qmp_cleanup_queue_and_resume.

monitor_resume does not need to be called from main context, so we can
call it immediately after popping a request from the queue, which allows
us to drop the qmp_queue_lock mutex before yielding.

Suggested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Message-Id: <20210322154024.15011-1-s.reiter@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>