Increase the helper VM's default RAM size to 512MB

The default Virtual RAM size for the helper VM used to be 256MB
Remove obsolete dependency to qemu-img

snf-image does not make any use of this utility any more
Remove chntpw and reglookup in favour of hivex*

Use hivexget and hivexregedit instead of reglookup and chntpw to browse
and alter the windows registry. The hivex utilities are better designed
and maintained