ci: move unit tests to KVM runner

Recent commits attempt to fix unit tests on a qubes runner, but there
are still some issues:

    FAIL: qubes.tests.app/TC_90_Qubes/test_112_default_guivm
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/user/builds/QubesOS/qubes-core-admin/qubes/tests/__init__.py", line 518, in cleanup_loop
        assert not self.loop._ready
    AssertionError
    ======================================================================
    FAIL: qubes.tests.app/TC_90_Qubes/test_112_default_guivm
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/user/builds/QubesOS/qubes-core-admin/qubes/tests/__init__.py", line 486, in cleanup_gc
        assert not leaked
    AssertionError

Move back to a KVM runner for now.

.gitlab-ci.yml

@@ -36,7 +36,7 @@ checks:tests:
   - PYTHONPATH=test-packages:~/qubes-core-qrexec ./run-tests
   stage: checks
   tags:
-  - vm
+  - vm-kvm
 include:
 - file: /r4.2/gitlab-base.yml
   project: QubesOS/qubes-continuous-integration