Revert "qmp: isolate responses into io thread"

This reverts commit abe3cd0f.

There is no need to add an additional queue to send the reply to the
IOThread, because QMP response is thread safe, and chardev write path
is thread safe. It will schedule the watcher in the associated
IOThread.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180829134043.31706-4-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>