blueprint host-aggregates: xenapi implementation

This commit introduces some clean-up/improvements on the current model
and api for host aggregates. It also introduces a first version of the
xenapi implementation. More precisely:

- it lays out the structure of the virt driver,
- it introduces compute and xenapi unit tests coverage,
- it deals with join/eject of pool master and slaves,
- it fixes xenapi_conn, when used in resource pool configurations

More commits to follow (to ensure that VM placement, networking setup,
performance metrics work just as well in cases where resource pools
are present). However, these may be outside the scope of this blueprint
and considered as ad-hoc bug fixes.

Change-Id: Ib3cff71160264c5547e1c060d3fd566ad87337cb

nova/exception.py

@@ -929,6 +929,11 @@ class QuotaError(NovaException):
     message = _("Quota exceeded") + ": code=%(code)s"
 
 
+class AggregateError(NovaException):
+    message = _("Aggregate %(aggregate_id)s: action '%(action)s' "
+                "caused an error: %(reason)s.")
+
+
 class AggregateNotFound(NotFound):
     message = _("Aggregate %(aggregate_id)s could not be found.")