tests/vm: pin ubuntu.i386 image

It's a good practice to always have the same components used in tests.
According to:

   https://cloud-images.ubuntu.com/releases/16.04/

New images are released from time to time, and the "release/"
directory points to the latest release.  Let's pin to the latest
available version, and while at it, set a hash for verification.
Signed-off-by: NCleber Rosa <crosa@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190613130718.3763-4-crosa@redhat.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

tests/vm/ubuntu.i386

@@ -61,7 +61,9 @@ class UbuntuX86VM(basevm.BaseVM):
         return os.path.join(cidir, "cloud-init.iso")
 
     def build_image(self, img):
-        cimg = self._download_with_cache("https://cloud-images.ubuntu.com/releases/16.04/release/ubuntu-16.04-server-cloudimg-i386-disk1.img")
+        cimg = self._download_with_cache(
+            "https://cloud-images.ubuntu.com/releases/16.04/release-20190605/ubuntu-16.04-server-cloudimg-i386-disk1.img",
+            sha256sum="e30091144c73483822b7c27193e9d47346dd1064229da577c3fedcf943f7cfcc")
         img_tmp = img + ".tmp"
         subprocess.check_call(["cp", "-f", cimg, img_tmp])
         subprocess.check_call(["qemu-img", "resize", img_tmp, "50G"])