VirtualBox is no emulator for N900 it is another platform where you can test maemo leste. If you would like to have N900 conditions you should use a N900.You have so many spares you could install to one maemo leste and compile dooble for it.
Or you wait until someone pops up who tested dooble on maemo leste and N900 together. Don't know the browser situation but I don't think that dooble is choosen to be the mobile browser on leste, thats why I guess that nobody tested it so far.