monitor: allow using IO thread for parsing

For each Monitor, add one field "use_io_thr" to show whether it will be
using the dedicated monitor IO thread to handle input/output.  When set,
monitor IO parsing work will be offloaded to the dedicated monitor IO
thread, rather than the original main loop thread.

This only works for QMP.  HMP will always be run on the main loop
thread.

Currently we're still keeping use_io_thr off always.  Will turn it on
later at some point.

One thing to mention is that we cannot set use_io_thr for every QMP
monitor.  The problem is that MUXed typed chardevs may not work well
with it now. When MUX is used, frontend of chardev can be the monitor
plus something else.  The only thing we know would be safe to be run
outside main thread so far is the monitor frontend. All the rest of the
frontends should still be run in main thread only.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-10-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: squash in Peter's followup patch to avoid test failures]
Signed-off-by: Eric Blake <eblake@redhat.com>