Control-Tab worked for me. Switches to the next VM tab to the right. Shift-Cntrl-Tab switches to the next VM tab to the left.
Edit: Ahh, I re-read your issue. No, i can't find a keystroke to switch focus back to the Library pane.
Sorry if I was unclear, but I'm not talking about moving between the tabs on the right. I'm talking about using the keyboard to set the focus to the Library pane on the left, so that I can use the keyboard to select a machine I want and open its tab. Interestingly enough, if you click in the Library pane on the left, you can then use the keyboard to navigate its contents, but then there doesn't seem to be any key to return the focus to the bunch of tabs on the right. It really seems like the developers have completely overlooked the need to be able to navigate between the library and the tabs using only the keyboard.