`multi-libvterm` function no longer accepts buffer name argument

[paulbdavis]

Since the last update was merged, I am no longer able to provide a buffer name to the multi-libvterm function. Looks like the argument was removed?

EDIT: Not really sure how this actually works, but just adding the optional argument back in and nothing else results in the previous behaviour.. I'm sure it's something I just don't understand about elisp (is the optional arg being passed through implicitly somhow??)

Sending a PR that reverts the behavior for review