Fix wrong description when updating quotas

when updating quota for a resource, with 'force_update=false', an
exception will be raised when the new value is less than the used quota
values(used and reserved). This patch changes the description of
exception.

Change-Id: Iede70444341817afb6953eb604fe4de186622916
Closes-bug: #1243062
xenapi: Fix regression issue in agent update

Agent updates must be processed after guest networking is configured

Change-Id: I7994154ea09befe07396b4e99d43157d268fd6b1
Closes-Bug: 1250162
Merge "xenapi: Fix regression issue in agent update"
Merge "Fix wrong description when updating quotas"