Temporary VM images now have -tmp- in their filename

Makefile

@@ -2,14 +2,14 @@ SHELL = bash -euET -o pipefail -c
 tmp_image := $(shell echo $$$$)
 
 all: hello.tar hello.sizes vm-image to-be-run-in-vm.sh Makefile
-	qemu-img create -f qcow2 -o backing_file=vm-image vm-image-${tmp_image}
+	qemu-img create -f qcow2 -o backing_file=vm-image vm-image-tmp-${tmp_image}
 # TODO: qcow2: make a derived image.
 	qemu-system-x86_64 -enable-kvm -m 256 \
-	  vm-image-${tmp_image} \
+	  vm-image-tmp-${tmp_image} \
 	  -drive format=raw,file=hello.sizes,if=ide,index=1,media=disk \
 	  -drive format=raw,file=to-be-run-in-vm.sh,if=ide,index=2,media=disk \
 	  -drive format=raw,file=hello.tar,if=ide,index=3,media=disk
-	rm vm-image-${tmp_image}
+	rm vm-image-tmp-${tmp_image}
 
 %.sizes: %.tar signing-key.pub to-be-run-in-vm.sh Makefile
 	printf "%020d\\n%020d\\n%$$((512-((20+1)*2)-1))s\\n" \