Prior hardware versions would always have the serial and parallel port hardware present within the virtual motherboard's SuperIO chip, even when there were no virtual serial ports or virtual parallel ports configured for the virtual machine -- the ports would quietly be "present but disconnected", so they would still consume I/O space and IRQs, and still show up as present in your guest OS's device list, I/O port map and IRQ map.
With the latest hardware version, if you remove the virtual serial port and virtual parallel port, they will be gone... the guest OS will not see the device at all, and it will consume no I/O port space and no IRQ.
Hope this helps!
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Darius