monitor: guard iothread access by mon->use_io_thread

monitor: guard iothread access by mon->use_io_thread

monitor_resume() and monitor_suspend() both want to
"kick" the I/O thread if it is there, but in
monitor_suspend() lacked the use_io_thread flag condition.
This is required when we later only spawn the thread on
first use.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180925081507.11873-2-w.bumiller@proxmox.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>