Add a test case that first transiently and then persistently unplugs a device

[scholzp]

This is a followup after merging BDF tests.

One edge-case would be to remove a device from the persistent config, after removing it from the transient config.
We could define the following hypothetical test case:

controllerVM.succeed(
    "qemu-img create -f raw /var/lib/libvirt/storage-pools/nfs-share/vdb.img 5M"
)
controllerVM.succeed(
    "virsh attach-disk --domain testvm --target vdb --source /var/lib/libvirt/storage-pools/nfs-share/vdb.img --persistent"
)
controllerVM.succeed("virsh detach-disk --domain testvm --target vdb")
controllerVM.succeed(
    "virsh attach-device testvm /etc/new_interface.xml --persistent "
)
controllerVM.succeed("virsh detach-disk --domain testvm --target vdb")
controllerVM.succeed("virsh detach-disk --domain testvm --target vdb --persistent")

Currently, the last command would fail. We need to think about what the expected outcome would look like, then improve libvirt and define a test case if necessary.