Remove unused elevated context param from quota helper methods

The elevated context being passed to the _quota_usage_create and
_reservation_create methods is not used, so remove it from the formal
parameter list of those methods and related usage.

This is part of a larger series to refactor the quota_reserve method.

Partial-Bug: #1369605

Change-Id: I50749402177eda29ae006ec4fd11a9a8125765ec
Add VIR_ERR_CONFIG_UNSUPPORTED to fakelibvirt

Commit 867bdedf81533f283aae4de4488d54c254bb7f07 added a test that raises
an exception using the libvirt VIR_ERR_CONFIG_UNSUPPORTED error code but
missed adding that to fakelibvirt, so the test won't work if you don't
have libvirt installed.

This simply adds the constant to fakelibvirt.

Closes-Bug: #1376363

Change-Id: I909608a7f1a8b5ee407b1d01896bc9db1adcee4e
Merge "Remove unused elevated context param from quota helper methods"
Merge "Add VIR_ERR_CONFIG_UNSUPPORTED to fakelibvirt"