When VM got some memory assigned, balloon driver may not pick it up
immediatelly and the memory will still be seen as "free" by Xen, but VM
can use (request) it at any time. Qmemman needs to take care of such
memory (exclude it from "free" pool), otherwise it would redistribute it
to other domains, allowing the original domain to drain Xen memory pool.
Do this by redefining DomainState.memory_actual - it is now amount of
memory available to the VM (currently used, or possibly used). Then
calculate free memory by subtracting memory allocated but not used
(memory_target-memory_current).
FixesQubesOS/qubes-issues#1389
181eb3e764