usb-device-manager: Avoid USB event thread leak
usb-device-manager: Avoid USB event thread leak

This is a follow-up of the previous commit ('usb-channel: Really stop
listening for USB events on disconnection').

Since the USB event thread has to be stopped when we destroy the
associated SpiceUsbDeviceManager, spice_usb_device_manager_dispose()
should force event_thread_run to FALSE even if
spice_usb_device_manager_stop_event_listening() was not enough. When
this happens, this means that there is a bug in the internal users of
spice_usb_device_manager_start_event_listening(), but with this change,
we'll at least warn about it, and avoid a thread leak/potential future
crash.

fgouget/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,flexVDI/spice-gtk