This will make it convenient to include these bots in Zulip API releases on pypi. Fix #5009.
Update example commands from calling by keyword 'fs' (As was done previously) to calling by at-mention of the bot. Updates '/contrib_bots/bots/virtual_fs/virtual_fs.py' by correcting instructions on how to call 'virtual_fs' bot.
Add test file 'test_virtual_fs.py'.