release: Build UFS and ZFS VM images

A new option 'VMFSLIST' controls the list of filesystems for which VM
images will be built; the default is to build both UFS and ZFS.

The vm-install target installs these as ${OSRELEASE}-${FS}.${FORMAT},
e.g. FreeBSD-14.0-CURRENT-amd64-zfs.vmdk. For backwards compatibility,
the ${VMFS} image is linked to the previously used ${OSRELEASE}.${FORMAT}
name.

Cloudware building will be updated in a later commit.

Reviewed by: gjb
Reviewed by: emaste, markj (previous version)
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41340