Welcome to the funhouse.
I think what you are saying is you use a virtualised WinXP to access itunes which in turn via the USB device updates the iphone.
Not sure about persistent USB devices. If you plug in the iphone, start virtualbox but not XP, select the XP machine and then the settings, I think the second green plus icon in USB filters may be what you need. Hover over the icon to read the help, and click it. With the iphone plugged in it should register it. Persistent? Not sure.