monitor: delay monitor iothread creation

monitor: delay monitor iothread creation

Commit d32749deb615 moved the call to monitor_init_globals()
to before os_daemonize(), making it an unsuitable place to
spawn the monitor iothread as it won't be inherited over the
fork() in os_daemonize().

We now spawn the thread the first time we instantiate a
monitor which actually has use_io_thread == true.
Instantiation of monitors happens only after os_daemonize().
We still need to create the qmp_dispatcher_bh when not using
iothreads, so this now still happens in
monitor_init_globals().

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Fixes: d32749deb615 ("monitor: move init global earlier")
Message-Id: <20180925081507.11873-3-w.bumiller@proxmox.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Peter Xu <peterx@redhat.com>
[This fixes a crash on shutdown with --daemonize]
Signed-off-by: Markus Armbruster <armbru@redhat.com>