hw/arm: Silence xlnx-ep108 deprecation warning during tests

The new deprecation warning for the xlnx-ep108 machine also pops up
during "make check" which is kind of confusing. Silence it if testing
mode is enabled.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Acked-by: Wei Huang <wei@redhat.com>
Message-id: 1510846183-756-1-git-send-email-thuth@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
