I don’t have any USB dongles for my XPS 13 to test with, but this Dell support pages seems to suggest the BIOS only supports some USB adapters. Perhaps you could manually download the image via linux usb stick over wifi?

[CODE]The XPS 13 with BIOS A05 or higher, supports PXE boot with LAN7500 chipset based USB NICs.[/CODE]
[URL=‘http://en.community.dell.com/techcenter/b/techcenter/archive/2012/07/25/dell-xps-driver-cabs-are-now-available.aspx’]http://en.community.dell.com/techcenter/b/techcenter/archive/2012/07/25/dell-xps-driver-cabs-are-now-available.aspx [/URL]