libqmi-glib: Use custom read_nonblocking() in qmi-endpoint-qmux

The default read_nonblocking() function in gpollableinputstream.c can hang indefinitely in an underlying call to poll() if the stream is_readable() function returns TRUE when the stream is not readable.

To resolve this, a custom read_nonblocking() function is added that calls poll() with a timeout and returns G_IO_ERROR_WOULD_BLOCK in case the function would risk blocking for an extended period of time.