tests/qtest: Check for the availability of virtio-ccw devices before using them

The devices might not have been compiled into the QEMU binary, so we
have to check before we can use them.

Message-Id: <20230512124033.502654-16-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
2 files changed