Versions of VirtualBox packaged on Neon and Fedora now have
a proper WM_CLASS:
Fedora: WM_CLASS(STRING) = "VirtualBox", "VirtualBox"
Neon: WM_CLASS(STRING) = "Qt-subapplication", "VirtualBox"
And install a matching virtualbox.desktop.
We can therefore drop this rule.
Side note: The rule should actually still have worked, but
for some reason KServiceTypeTrader refuses to match a .desktop
with 'Exec=VirtualBox %U' when we query for 'VirtualBox', as
extracted from cmdline. I'm talking to dfaure about it.