kani: stop using deprecated cli args

--enable-unstable and --restrict-vtable got deprecated in kani 0.59.0 in
favor of variants based on -Z flags. So use the -Z flags instead

(cherry picked from commit 8495af9)
Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

tests/integration_tests/test_kani.py

@@ -29,12 +29,12 @@ def test_kani(results_dir):
     """
     # -Z stubbing is required to enable the stubbing feature
     # -Z function-contracts is required to enable the function contracts feature
-    # --restrict-vtable is required for some virtio queue proofs, which go out of memory otherwise
+    # -Z restrict-vtable is required for some virtio queue proofs, which go out of memory otherwise
     # -j enables kani harnesses to be verified in parallel (required to keep CI time low)
     # --output-format terse is required by -j
-    # --enable-unstable is needed to enable `-Z` flags
+    # -Z unstable-options is needed to enable the other `-Z` flags
     _, stdout, _ = utils.check_output(
-        "cargo kani --enable-unstable -Z stubbing -Z function-contracts --restrict-vtable -j --output-format terse",
+        "cargo kani -Z unstable-options -Z stubbing -Z function-contracts -Z restrict-vtable -j --output-format terse",
         timeout=TIMEOUT,
     )