virtio: Fail if iommu_platform is requested, but unsupported

virtio: Fail if iommu_platform is requested, but unsupported

Commit 2943b53f6 (' virtio: force VIRTIO_F_IOMMU_PLATFORM') made sure
that vhost can't just reject VIRTIO_F_IOMMU_PLATFORM when it was
requested. However, just adding it back to the negotiated flags isn't
right either because it promises support to the guest that the device
actually doesn't support. One example of a vhost-user device that
doesn't have support for the flag is the vhost-user-blk export of QEMU.

Instead of successfully creating a device that doesn't work, just fail
to plug the device when it doesn't support the feature, but it was
requested. This results in much clearer error messages.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1935019
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20210429171316.162022-6-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>