virt-sandbox: document how to pass arguments to commands
virt-sandbox: document how to pass arguments to commands

The example was already correct but the synopsis didn't mention '--' is
required.

agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,libvirt/libvirt-sandbox