build-system: add coverage-report target

build-system: add coverage-report target

This will build a coverage report under the current directory in
reports/coverage. At the users option a report can be generated by
directly invoking something like:

  make foo/bar/coverage-report.html

Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>