Pulseaudio Device provider doesn't handle devices hotplug

Hi It worked with gstreamer 1.14.5 but doesn't work now.

I did a few tests.

• Installed Gstreamer 1.16.2 on Gentoo Linux
• Started pavucontrol and made sure it reacts properly to my usb headphone hot-plug
• Started gst-device-monitor-1.0 -f and tried to plug/unplug same usb headphone.
  • For some reason gst-device-monitor-1.0 shows no events for my headphone, but it shows events for other devices (reindexed?)
• Wrote a custom application which listens on devices bus and calls gst_device_monitor_get_devices on any event dumping list to terminal
  • The dumped list never changes on plug/unplug and my headphone is there only if it was initially plugged-in.
  • GST_MESSAGE_DEVICE_ADDED/GST_MESSAGE_DEVICE_REMOVED unexpectedly were never received. just GST_MESSAGE_DEVICE_CHANGED.

The headphone is based on CM108 chip.