Volume detection in the scsi backend was duplicating code already
present in storage_backend.c. Let's drop the duplicate code.
Also, change the shared function name to be less generic, and remove
some error squashing in the other call site.
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
