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

release/Makefile.vm

@@ -50,8 +50,8 @@ vm-${_CW:tl}:
 
 .if defined(WITH_VMIMAGES) && !empty(WITH_VMIMAGES)
 CLEANDIRS+=	${VMTARGETS}
-CLEANFILES+=	${VMBASE}.img
 . for FORMAT in ${VMFORMATS}
+CLEANFILES+=	${FORMAT}.img
 CLEANFILES+=	${VMBASE}.${FORMAT}
 . endfor
 .endif
@@ -65,7 +65,7 @@ vm-image:
 	env TARGET=${TARGET} TARGET_ARCH=${TARGET_ARCH} \
 		${.CURDIR}/scripts/mk-vmimage.sh \
 		-C ${.CURDIR}/tools/vmimage.subr -d ${.OBJDIR}/${.TARGET} \
-		-i ${.OBJDIR}/${VMBASE}.img -s ${VMSIZE} -f ${FORMAT} \
+		-i ${.OBJDIR}/${FORMAT}.img -s ${VMSIZE} -f ${FORMAT} \
 		-S ${WORLDDIR} -o ${.OBJDIR}/${VMBASE}.${FORMAT}
 . endfor
 .endif