Use Versaloon as a CDC device

Information:
    Versaloon firmware is based on VCOM example from ST, it uses standard CDC class of USB.
    Versaloon can support as high as 912600 bps for UART interface. And it enables double buffer on both TX and RX to get the max performance.
    There are both 1KBytes TX ring buffer and 1KBytes RX ring buffer. It will NACK to PC if the TX buffer is full. For example, a single write with 100Kbytes date is called. Versaloon will first fill the buffer, after the buffer is full, it will NACK the USB transaction. So PC will re-send the USB data, until the free buffer is enough for the data and Versaloon returns ACK. If RX buffer is full(data is received from UART, but PC doesn't read it), the red led will be lighted.

Under Linux:
    After SVN533, the default cdc_acm kernel driver is used to drive Versaloon before you run  vsprog or openocd. So you simply connect Versaloon, and use /dev/ttyACMx. After vsprog or openocd is run, Versaloon will be detached from kernel cdc_acm driver.

Under Windows:
    Detaching from kernel driver under Windows is not supported. So you'll have to install Versaloon using stmcdc.inf as driver. stmcdc.inf is the driver for VCOM example in STM32_USB-FS-Device library from ST. If the driver is well installed, "STM32 Virtual COM Port(COMx)" will appear under "COM and LPT". If the old driver is installed for Versaloon, remove it in device mananger first.