virttest.bootstrap:never try to change the list when traverse it

Currently, when create config files, autotest try to find out
the config files need to overridden to the testdir/cfg/*.cfg.

But it trys to pop the element for the list during traversing
itself, this is very dangous, and will cause fault when run
get_start.py, because the list has update yet, but the index
for traverse hasn't update.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>