add option to vm.py to not mount or format a persistent image.
add option to vm.py to not mount or format a persistent image.

DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc