xenapi: vm_utils.ensure_free_mem does not take into account overheads

[John Garbutt]

While checking the free memory is always racey, we can do better.

We should use the memory overhead estimates introduced here:
https://review.openstack.org/#/c/35380

This should let us spot when a scheduler has incorrect placed a VM request on host much earlier, so reduce the time penalty of a re-schedule.

[Joe Gordon (jogo)]

is this still valid?

[John Garbutt (johngarbutt)]

yes, its still valid, just not fixed yet.

[Markus Zoeller (markus_z) (mzoeller)]

This is an automated cleanup. This bug report has been closed because it
is older than 18 months and there is no open code change to fix this.
After this time it is unlikely that the circumstances which lead to
the observed issue can be reproduced.

If you can reproduce the bug, please:
* reopen the bug report (set to status "New")
* AND add the detailed steps to reproduce the issue (if applicable)
* AND leave a comment "CONFIRMED FOR: <RELEASE_NAME>"
  Only still supported release names are valid (LIBERTY, MITAKA, OCATA, NEWTON).
  Valid example: CONFIRMED FOR: LIBERTY