tests/ivshmem-test: Supply missing initializer in get_device()

If the device isn't found, the assertion uses dev without
initialization.  Fix that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1448384789-14830-4-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
