vfio: Reset vfio_container in detaching last group

[minwooim]

If application follows the calling sequence, it fails in getting device fd as the following logs, Assumming that there's only one vfio group and container with a single device attached.

nvme_ctrl_init (e.g., unvme add )
nvme_close (e.g., unvme del )
nvme_ctrl_init

I nvme_pci_init (src/nvme/core.c:510): nvme/core: pci class code is 0x010802
I vfio_get_device_fd (src/iommu/vfio.c:430): iommu/vfio: vfio iommu group is /dev/vfio/5
I vfio_group_set_container (src/iommu/vfio.c:290): iommu/vfio: adding group '/dev/vfio/5' to container
D vfio_get_device_fd (src/iommu/vfio.c:438): iommu/vfio: failed to get device fd
D vfio_pci_open (src/vfio/pci.c:151): vfio/pci: failed to get device fd

This is because kernel removes container->iommu_driver by setting it to NULL if this is the last vfio group to be detached from the current container. This will fail in the following vfio kernel code:

int vfio_group_use_container(struct vfio_group *group) {
lockdep_assert_held(&group->group_lock);

    /*
     * The container fd has been assigned with VFIO_GROUP_SET_CONTAINER but
     * VFIO_SET_IOMMU hasn't been done yet. */ if (!group->container->iommu_driver) return -EINVAL;

...

VFIO_SET_IOMMU ioctl is required to re-initailize container->iommu instance. But, current libvfn is skipping vfio_iommu_type1_init in case vfio->iommu_set is set.

This patch resets the vfio_container instance when the last vfio group is detached from the current container just like kernel does.

[birkelund]

Looks good @minwooim. Can you force push to trigger the device tests? I added the Approved label so they run.