IF you are so inclined though(and I have done this in the past) you can use a +5 psu(make one, or radio shack should have something) and just add that. AFAIK the definitions of USB Slave vs. USB Host are different in 2 ways.
1) USB Host supplies a +5 and GND lead for the USB Host/Slave on the other end.
2) USB Host has code/firmware written that knows to do more than 'listen on the line'
Sorry I can't be more technical, but my experiences in usb device creation is limited.