release: Use qemu when cross-building vm images

For a bit over 5 years, we have used qemu when cross-building cloudware
images; in particular, it's necessary when installing packages which
might include post-install scripts.

Use qemu in the vm-images target too; while "generic" vm images don't
install packages, they still run newaliases and /etc/rc.d/ldconfig,
both of which fail without appropriate emulation.

MFC after:	1 week

release/Makefile.vm

@@ -157,12 +157,13 @@ CLEANFILES+=	${VMBASE}.${FS}.${FORMAT}
 
 vm-base:	vm-image
 
-vm-image:
+vm-image:	${QEMUTGT}
 .if defined(WITH_VMIMAGES) && !empty(WITH_VMIMAGES)
 . for FORMAT in ${VMFORMATS}
 .  for FS in ${VMFSLIST}
 	mkdir -p ${.OBJDIR}/${.TARGET}-${FORMAT}-${FS}
 	env TARGET=${TARGET} TARGET_ARCH=${TARGET_ARCH} SWAPSIZE=${SWAPSIZE} \
+		QEMUSTATIC=${QEMUSTATIC} \
 		${.CURDIR}/scripts/mk-vmimage.sh \
 		-C ${.CURDIR}/tools/vmimage.subr \
 		-d ${.OBJDIR}/${.TARGET}-${FORMAT}-${FS} -F ${FS} \