vdpa: Use helper for safer setting of driver_override

Use a helper to set driver_override to the reduce amount of duplicated
code.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220419113435.246203-9-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>