release: Stage non-UFS images in vm-images-stage

When the VM image building code was updated to support building
non-UFS images, the vm-images-stage target was not updated to
install those newly built images to the FTP site. As a result, we
have been sending weekly snapshot announcements since August claiming
that ZFS VM images are available when they are not in fact present
anywhere publicly accessible.

Fixes: 32ae9a6b3937 "release: Build UFS and ZFS VM images"
Reported by: Michael Dexter
MFC after: 5 days