Enable rust-vmm coverage test in CI

1. Enabled the aarch64 coverage test in `test_coverage` script.

2. Seperated the coverage json sample file to two sample files for
different machine achitectures.

3. Updated Buildkite pipeline container version.

4. Updated README for the description of coverage files and container
version.

Signed-off-by: Henry Wang <henry.wang@arm.com>

coverage_config_x86_64.json.sample

@@ -0,0 +1,5 @@
+{
+  "coverage_score": 90,
+  "exclude_path": "my_autogenerated_file.rs,second_file.rs",
+  "crate_features": "my_dummy_feature,dummy_feature2"
+}