[Libguestfs] [PATCH] sysprep: Enable the network, add --no-network to disable it

This is just one possible way to fix this bug, but the simplest. Others I considered: - Keeping the network disabled by default. Options like --install wouldn't work and there would be no actionable error message telling users what to do to fix it. - Modifying virt-customize to warn/error/suggest if operations like --install were used but the network was disabled. However it's not always wrong to use --install + --no-network, so getting the right message is difficult. - Disabling any customize operations that are not relevant to virt-sysprep. I think this is unnecessarily harsh. Rich.