With the information provided thus far the only suggestions I can make is to either install the software for the PLC in the Windows XP Virtual Machine or to physically dual-boot Windows 7 and Windows XP where the dongle can work directly with the natively installed Windows XP OS. If it's a case of the software requires Windows 7 then see if you can acquire a different dongle that has working Windows 7 Drivers! 
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Re: How to connect a serial port on the host to a serial port on the VM?
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