qemu: Alter VM Generation ID for specific startup/launch transitions

qemu: Alter VM Generation ID for specific startup/launch transitions

Before we generate the command line for qemu, if the domain about to
be launched desires to utilize the VM Generation ID functionality, then
handle both the regenerating the GUID value for backup recovery (restore
operation) and the startup after snapshot as both require a new GUID to
be generated to allow the guest operating system to recognize the VM
is re-executing something that has already executed before.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>