XenAPI: Change the default SR to be the pool default

The default SR should be the pools default - but it might be unset
if a host has been removed from a host aggregate (i.e. removed from
the pool).  In this case make sure that we do give a decent error
message.

Change-Id: I116f65ece94f839e63b085480ba4f8974119d2bb
Fixes: bug 1198239