The sancov filter check still fails when unused arguments are treated as
errors. To work around that, add a SanitizerCoverage flag to the
build-check.

Fixes: aa4f3a3b ("build: fix check for -fsanitize-coverage-allowlist")
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20220621204507.698711-1-alxndr@bu.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>