tests: Add an option for snapshot (default: off)

Not using snapshot has the benefit of automatically persisting useful
test harnesses, such as docker images and ccache database. Although it
will lose some cleanness, it is imaginably useful for patchew.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20180712012829.20231-2-famz@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/vm/basevm.py

@@ -217,6 +217,8 @@ def parse_args(vm_name):
                       help="build QEMU from source in guest")
     parser.add_option("--interactive", "-I", action="store_true",
                       help="Interactively run command")
+    parser.add_option("--snapshot", "-s", action="store_true",
+                      help="run tests with a snapshot")
     parser.disable_interspersed_args()
     return parser.parse_args()
 
@@ -242,7 +244,10 @@ def main(vmcls):
                    jobs=args.jobs)]
         else:
             cmd = argv
-        vm.boot(args.image + ",snapshot=on")
+        img = args.image
+        if args.snapshot:
+            img += ",snapshot=on"
+        vm.boot(img)
         vm.wait_ssh()
     except Exception as e:
         if isinstance(e, SystemExit) and e.code == 0: