tests: vm: Add vm-clean-all

The images are big. Add a rule to clean up easily.
Suggested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20180716020008.31468-1-famz@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/vm/Makefile.include

 # Makefile for VM tests
 
-.PHONY: vm-build-all
+.PHONY: vm-build-all vm-clean-all
 
 IMAGES := ubuntu.i386 freebsd netbsd openbsd centos
 IMAGE_FILES := $(patsubst %, tests/vm/%.img, $(IMAGES))
@@ -18,6 +18,9 @@ vm-test:
 
 vm-build-all: $(addprefix vm-build-, $(IMAGES))
 
+vm-clean-all:
+	rm -f $(IMAGE_FILES)
+
 tests/vm/%.img: $(SRC_PATH)/tests/vm/% \
 		$(SRC_PATH)/tests/vm/basevm.py \
 		$(SRC_PATH)/tests/vm/Makefile.include