make: Clean up coverage files
make: Clean up coverage files

Running `make go-coverage` (e.g. as part of `make travis`) creates a number
of .html and .cov files that `make clean` wouldn't clean up. We now give the
output coverage files the .cov.html extension, so `make clean` can easily
remove these files without removing the unrelated .html files (e.g. in the docs
and integration tests).

NetSys/quilt,LuiseV/quilt,kklin/quilt,quilt/quilt,kelda/kelda,kklin/kelda,NetSys/quilt,quilt/quilt,NetSys/quilt,NetSys/quilt,kelda/kelda,kelda/kelda,quilt/quilt,kklin/quilt,quilt/quilt,kklin/kelda,kklin/kelda,kelda/kelda,NetSys/quilt,kklin/quilt,LuiseV/quilt,kklin/quilt,LuiseV/quilt,LuiseV/quilt,kklin/kelda,LuiseV/quilt