[platforms/virtual] Create output devices

Summary:
Create output devices in virtual backend. For that the setVirtualOutputs call
can only come after the Wayland server has been initiliazied such that the
display exists to create the output and output device interfaces. Tests have
been adjusted for that.

Test Plan:
```
98% tests passed, 3 tests failed out of 148

Total Test time (real) = 362.97 sec

The following tests FAILED:
         33 - kwin-testInternalWindow (Failed)
         39 - kwin-testPointerInput (Failed)
        101 - kwin-testMoveResize (Failed)
```
Failing of these tests looks unrelated to the change.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23477