use ballooning to improve snapshot performance
https://github.com/os-autoinst/os-autoinst/pull/1020 allows to enable memory ballooning to improve snapshot performance using the parameter "QEMU_BALLOON_TARGET". We can set this parameter and try out what impact on performance it has.
Discussed with rpalethorpe. The variable "QEMU_BALLOON_TARGET" should be set to a reasonable value in the range [0;QEMURAM] in MB. The higher the value, the higher is the amount of RAM that is "requested" for ballooning, i.e. how much (free) RAM space from the VM should be consumed and hence reclaimed. The higher the value the better as snapshots will decrease accordingly but also the longer it can take for the reclaiming which can eventually timeout. The theoretical upper limit is the actually used RAM within the VM.