tests: Fix setup_machine.sh script for diskless machines

This commit fixes the setup_machine.sh script to make it possible to
call it on machines which don't have any hard drives, because the
following:

  ./setup_machine.sh setup

was failing with:

  ./setup_machine.sh: line 145: loops[*]: unbound variable

Change-Id: I254ecd5fe8e377f013e82aba725b5b5fcaf8b38d