[libvirt] Add virt-sandbox -s inherit, to execute the sandbox with parents label

[Dan Walsh]

This will allow us to run sandbox as the calling process,  If I am
running a shell as staff_u:unconfined_r:unconfined_t:s0, and I
execute virt-sandbox -c lxc/// -- /bin/sh

The second patch fixes a problem when users try to upgrade Generic Containers.

[sandbox PATCH 1/2] Add virt-sandbox -s inherit, to execute the
[sandbox PATCH 2/2] GenericContainers do not have unit files.