main-loop: update comment for qemu_mutex_lock/unlock_iothread

Commit 49cf57281b7 (vl: delay thread initialization after daemonization)
makes the global mutex is taken after daemonization instead before
daemonization by qemu_init_main_loop().

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Message-Id: <1480566640-27264-2-git-send-email-baiyaowei@cmss.chinamobile.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
