I haven't set up virtual xp on a Windows 7 box, so all I can tell you right now is something of a hack.
In the XP system, edit C:\Windows\System32\drivers
\etc\hosts with notepad.
Add a line at the bottom:
<your IP> name_of_the_win7_host
Obviously, since you're putting in the IP, you'll have to keep it current, but this at least adds a level of indirection so you can edit it in one place but refer to the name wherever you want.
I'll check around to see what I can find for a more elegant solution in the meantime.
Greg Oliver [MSFT]