run-coverity-scan: add --check-upload-only option

run-coverity-scan: add --check-upload-only option

Add an option to check if upload is permitted without actually
attempting a build.  This can be useful to add a third outcome
beyond success and failure---namely, a CI job can self-cancel
if the uploading quota has been reached.

There is a small change here in that a failure to do the upload
check changes the exit code from 1 to 99.  99 was chosen because
it is what Autotools and Meson use to represent a problem in the
setup (as opposed to a failure in the test).

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>