Reduce the default image size for virtual machine disk images from
30GB to 3GB. The raw images can be resized using truncate(1), and
other formats can be resized with tools included with other tools
included with other hypervisors.

Enable the growfs(8) rc(8) at firstboot if the disk was resized
prior to booting the virtual machine for the first time.

Discussed with: several
PR: 232313 (requested in other context)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation