iotests/163: Fix broken qemu-io invocation

The 'read' commands to qemu-io were malformed, and this invocation only
worked by coincidence because the error messages were identical. Oops.

There's no point in checking the patterning of the reference image, so
just check the empty image by itself instead.

(Note: as of this commit, nothing actually enforces that this command
completes successfully, but a forthcoming commit in this series will
enforce that qemu_io() must have a zero status code.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220418211504.943969-3-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

tests/qemu-iotests/163

@@ -113,10 +113,7 @@ class ShrinkBaseClass(iotests.QMPTestCase):
         qemu_img('resize',  '-f', iotests.imgfmt, '--shrink', test_img,
                  self.shrink_size)
 
-        self.assertEqual(
-            qemu_io('-c', 'read -P 0x00 %s'%self.shrink_size, test_img),
-            qemu_io('-c', 'read -P 0x00 %s'%self.shrink_size, check_img),
-            "Verifying image content")
+        qemu_io('-c', f"read -P 0x00 0 {self.shrink_size}", test_img)
 
         self.image_verify()