tests: run 'device-crash-test' from tests/venv

Remove the sys.path hacking from device-crash-test, and add in a little
user-friendly message for anyone who was used to running this script
directly from the source tree.

Modify the GitLab job recipes to create the tests/venv first, then run
device-crash-test from that venv.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-10-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>