storage: Don't update pool available/allocation if buildVol fails

storage: Don't update pool available/allocation if buildVol fails

https://bugzilla.redhat.com/show_bug.cgi?id=1024159

If adding a volume to a storage pool fails during the CreateXML or
CreateXMLFrom API's, we don't want to adjust the available and
allocation values for the storage pool during storageVolDelete
since we haven't adjusted the values for the create.

Refactor storageVolDelete() a bit to create a storageVolDeleteInternal()
which will handle the primary deletion activities.  Add a parameter
updateMeta which will signify whether to update the values or not.

Adjust the calls from CreateXML and CreateXMLFrom to directly call the
DeleteInternal with the pool lock held.  This does bypass the call
to virStorageVolDeleteEnsureACL().