core: RemoveStoragePool - operations order

It makes more sense to attempt to diconnect from the StoragePool before
disconnecting from the storage server as different operations might
be affected (for example -  domain monitoring thread).

Change-Id: I67e801df32f330d7b89bab6a189a5a15fa119f68
Bug-Url: https://bugzilla.redhat.com/1059769
Signed-off-by: Liron Aravot <laravot@redhat.com>