Make gzipping disk images optional (default on)

[mauiaaron]

Automatically gzipping disk images should be a user choice. The default should be gzipped.

[mauiaaron]

Disk image access behavior is changing.

• images accessed readonly will not be changed at all, even if they are gzipped (previously we would gunzip/gzip them in-place, but this is potentially destructive)
• images accessed read/write will continue to be gunzipped/gzipped in place at no extra cost