ADB over TCPIP connects, but no command works after connection has been done
After 3-4 days of trial-error I've found what's the source of the issue...

It has something to do with virtualized ethernet adapter hardware. When I switched to Virt IO and I installed the driver in my Windows 7 guest machine, ADB has started to work as expected!

Conclusion: if you want to debug Android apps from a Windows guest to an emulator or device, configure you VirtualBox machine to use VirtIO (paravirtualized) network and you'll be fine!