Remove used_limits extension from the V3 API

Remove the used_limits extension from the V3 API because
it shows usage for limits which are no longer going to be displayed
by the limits extension. See https://review.openstack.org/#/c/39872/
The limits are not being displayed there because they are already
available through the quotas extension.

Although the quotas extension currently only displays the quota limit
and not usage, the following proposed changeset adds display
of usage: https://review.openstack.org/#/c/32303/. And this is done
in a way which is much more consistent in terms of naming and interface
than used_limits does.

So we no longer need the used_limits extension

Partially implements blueprint nova-v3-api

Change-Id: I8ec1f300335ca4fc3b86323cc3af6bc1591da7bb
ComputeFilter shouldn't generate a warning for disabled hosts

Disabling a service is a normal operation, so this state should
not be included in the same log statement as a host that fails
the service_is_up() check.

A service which is enabled but not up deserved a warning rather
than debug entry.

By separating out the checks we also avoid the service check on
disabled hosts.

Change-Id: I1a398bc033ebe71f42ac117af7dae39502aceede
Closes-Bug: 1254356
There is no need to set VM status to ERROR on a failed migration

VM state should not be set to ERROR, when the
migration failed beauce libvirt is unavailble on the destination.

Fixes bug #1233184
Change-Id: I36e4ed3842d7e33c1082ec95f860629eee23224e
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 "There is no need to set VM status to ERROR on a failed migration"
Merge "ComputeFilter shouldn't generate a warning for disabled hosts"
Merge "Remove used_limits extension from the V3 API"
Merge "xenapi: Update VM memory overhead estimation"