FreeBSD VM/cloud images tend to be highly compressible: The generic VM
images compress roughly 3.5:1, and cloud images typically even more
since they have significant unused space in their virtual disks. This
generally doesn't matter for users who can download compressed release
images and extract them locally, but for EC2 in particular relying on
uncompressed image formats is painful: We now upload over 500 GB/week
to AWS.

This commit adds the "stream-optimized" version of the VMDK format,
which compresses each 64 kB "grain" individually (and omits grains
which are all zeroes). Compared to uncompressed formats, this can
produce much smaller images; experiments indicate that using this
format for EC2 image uploads will reduce the weekly traffic to under
100 GB/week.

Co-authored-by: Claude Opus 5.5
Sponsored by: Amazon