Unable to disable gvnc dependency during build
Description of problem:
The qtest tests will pick up a copy of gvnc if it happens to be installed and there does not appear
to be any way of disabling the dependency to ensure a reproducible build. We tripped over this in
bulk builds on OpenBSD.
Steps to reproduce:
1. Install gvnc
2. Build QEMU
Additional information:
From tests/qtest/meson.build

```
if vnc.found()
  gvnc = dependency('gvnc-1.0', method: 'pkg-config', required: false)
  if gvnc.found()
    qtests += {'vnc-display-test': [gvnc]}
    qtests_generic += [ 'vnc-display-test' ]
  endif
endif
```