When iterating through VMFORMATS, the VMBASE file is
not removed or truncated to a zero-size file, which
if used to create more than one disk image format, can
result in accidental pollution of the target formatted
disk image.

Instead of using a single VMBASE image (vm.img, by
default), use a single base file for each format, named
as VMFORMAT.img, which produces VMBASE.VMFORMAT as the
final formatted image.

Reported by: cperciva
MFC after: 1 month
X-MFC-with: r277458, r277536
Sponsored by: The FreeBSD Foundation