I would like to know if it's safe to update the firmware of my iPhone 4S through a Windows XP virtual machine created on VMWare Workstation 8 with forwarded USB.
Short and simple answer:
Having said that I have to admit that I have done firmware upgrades through a Virtual Box Windows guest (though not with an iPhone) without any problems. It may work fine in most cases. But as soon as the VM has proplems related to the virtualization you may end up with an incomplete firmware upgrade that may render your device unusuable.
Therfore it is recommended and much safer to upgrade a firmware by using another machine running with the OS the upgrade software is designed for.