kvm: Minimise restart reaction of the KVM wrapper

Thanks to minimising amount of changing parameters of the wrapper, by setting
RAM and CPU to "init" values, it's possible to not restart the wrapper in case
of device hotplugging in for changes of those parameters.

Note that this shows regressiong to enable-device-hotplug which happened
after upgrade to qemu 5.2.0 in 8dc22418.