The virConnectPtr is no longer required for error reporting since
that is recorded in a thread local. Remove use of virConnectPtr
from all APIs in storage_conf.{h,c} and storage_encryption_conf.{h,c}
and update all callers to match
Allows the initiator to use a variety of IQNs rather than just the
system IQN when creating iSCSI pools.
* docs/schemas/storagepool.rng: extends the syntax with <iqn name="..."/>
* src/conf/storage_conf.[ch]: read and stores the iqn name
* src/storage/storage_backend_iscsi.[ch]: implement the IQN selection
when detected
