machines: start vm checkbox is too large

[garrett]

The "Immediately Start VM" checkbox is a bit too large:

This is due to it pulling in the classes "form-control dialog-checkbox" that set height and width oddly (for a checkbox) on the #start-vm input.

Solutions (choose one):

• Remove form-control and dialog-checkbox on the input element's class attribute

• If you need the classes: in pkg/machines/machines.less, change height and width to unset, like so:

  .dialog-checkbox {
    width: unset;
    height: unset;
  }

  (Unset resets the CSS value to default.)

[martinpitt]

I don't see that either class is actually necessary at runtime or for tests, so I just dropped these. Fix sent in PR #9177.