virtio-iommu: Fix virtio_iommu_mr()

Due to an invalid mask, virtio_iommu_mr() may return the wrong memory
region. It hasn't been too problematic so far because the function was
only used to test existence of an endpoint, but that is about to change.

Fixes: cfb42188 ("virtio-iommu: Implement attach/detach command")
Cc: QEMU Stable <qemu-stable@nongnu.org>
Acked-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Message-Id: <20201030180510.747225-2-jean-philippe@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>