virtio-iommu: Fix coverity issue in virtio_iommu_handle_command()

virtio-iommu: Fix coverity issue in virtio_iommu_handle_command()

Coverity points out (CID 1430180) that the new case is missing
break or a /* fallthrough */ comment. Break is the right thing to
do as in that case, tail is not used.

Fixes 1733eebb9e ("virtio-iommu: Implement RESV_MEM probe request")
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200708160147.18426-1-eric.auger@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>