This validates that the rust code in src/ references all
the funtions, macros and enums defined in the libvirt API
XML document.

Due to large gaps we cannot yet validate the full API
coverage. Thus we define various name prefixes under
which we enforce full coverage. The rest is only
validated if enabling the 'api_coverage' feature, eg

  cargo test --feature api_coverage

Initially the virSecret, virError, VIR_FROM & VIR_ERR
prefixes are marked as enforcing since we have 100%
API coverage for them.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>