[platforms/virtual] Make VirtualOutput non-copyable

Summary:
Qt objects are treated as identities so remove the copy constructor and
an assignment operator. See, http://doc.qt.io/qt-5/object.html#identity-vs-value

IIRC some Qt objects(like QImage) have copy constructor for implicit data sharing,
but VirtualOutput doesn't have anything to share.

Also, it [this patch] fixes a compiler warning:

```
/home/vlad/KDE/src/kde/workspace/kwin/plugins/platforms/virtual/virtual_output.cpp: In copy constructor ‘KWin::VirtualOutput::VirtualOutput(const KWin::VirtualOutput&)’:
/home/vlad/KDE/src/kde/workspace/kwin/plugins/platforms/virtual/virtual_output.cpp:30:1: warning: base class ‘class QObject’ should be explicitly initialized in the copy constructor [-Wextra]
 VirtualOutput::VirtualOutput(const VirtualOutput &o)
 ^~~~~~~~~~~~~
```

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: michaelh, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D12214