[libvirt] [RFC] add option to bring new qemu binary/pending conf changes on reboot

Hi, all. I'm trying to reword the long pending RFC [1]. We interested in next usecase. Administrator update qemu binary/make domain configuration changes that transparent to the guest. This changes are not applied until domain shutdown/started by domain user. Sometimes user just reboots domain and these changes as they transparent can be applied too and this is a good opportunity to apply changes. For example new qemu binary have some optimizations and administrator would like to use these optimizations and don't want to force user to restart, waiting for user reboot is considered good enough. To support this use case we can add 'recreate' option to on_reboot/on_shutdown event. [1] https://www.redhat.com/archives/libvir-list/2016-June/msg02259.html