iTunes library for multiple machines
The best way to go, in my opinion, for a flawless and easy experience would be iTunes Match. It doesn't only allow you to play your tunes on iOS devices, but also on other desktop computers running iTunes, and it Syncs everything via the cloud, including all your metadata, so basically you'd have your iTunes library wherever you have internet. 

It costs 25 USD a year (plus extra storage, if you have more than 5GB of songs that are not in the iTunes Store), though if you can spare it you not only get the syncing, but also the upgrade of all your tunes to 256kbps ACC sound.

Home sharing also works, and does update your play counts if you enable it to under Preferences > Sharing, but it will pretty much just open a "read-only" version of your library on the rest of the computers; you cannot edit metadata, change rating, or even browse your library in any but the list view mode, plus of course, if the computer disconnects from the network it looses all access to the music, for Home Sharing makes no local copy in each machine.

Other methods should work, I believe, to share your music, but yeah, I remember I tried some time ago and I didn't manage to keep the iTunes Library file and thus data like play counts in sync.

Btw, I believe the file iTunes Library.itl is the main resource for the Library Database, and the iTunes Music Library.xml file is just a "backup" and a way for other applications to access data of the library if they need it, so you would need to keep in the .itl file in sync.

Good Luck!

Kevin