v4l2: Workaround libv4l2 RW emulation bug
When libv4l2 emulates RW mode on top of MMAP devices, the queues are only initialized on first read. The problem is that poll() will fail if called before the queues are initialized and streaming. Workaround this by doing a zero size read when pool is started in that IO mode. https://bugzilla.gnome.org/show_bug.cgi?id=740633