xenapi: Update VM memory overhead estimation

Update the VM memory overhead estimation formula to be more accurate
based on a more comprehensive survey of VM overhead values found in
the Rackspace public cloud's XenServer hosts.

The old formula was under-predicting overhead in some cases, especially
when VMs had a larger number of VCPUs.

Closes-Bug: 1257874

Change-Id: I338c72145fe41dae094516f7f832f15efc46a530
Merge "xenapi: Update VM memory overhead estimation"