I can confirm this bug too. 'qemubuilder --create' (and presumably '--build' or '--login' too) cannot complete succesfully without this being fixed. A patch adding the missing arguments follows.
-- Benoît Knecht
diff --git a/qemubuilder.c b/qemubuilder.c index fb26dee..2086f09 100755 --- a/qemubuilder.c +++ b/qemubuilder.c @@ -315,6 +315,10 @@ static int fork_qemu(const char* hda, const char* hdb, const struct pbuilderconf argv[argc++]=append_command; argv[argc++]="-serial"; argv[argc++]="stdio"; + argv[argc++]="-net"; + argv[argc++]="user"; + argv[argc++]="-net"; + argv[argc++]="nic"; argv[argc]=NULL; assert(argc < MAX_ARGS);