Protobuf example should use a Python virtualenv

`protobuf` example is installing a package into the Python environment, in order to not pollute the main one it should be installed into a virtualenv (some thoughts [here](https://github.com/conan-io/examples/pull/49#pullrequestreview-355284866))

What's the problem? if the package protobuf is already installed, after running the example it will be removed.