As reported on the devel mailing list by Christos Stavrakakis,
creation of plain instances is broken when the --no-wait-for-sync flag
is passed, because in that case WaitForSync is not called, hence
SetDiskID is not called at all, resulting in a None physical_id being
passed to backend.

We fix that by explicitly calling SetDiskID, which will cover the
pause/resume and os_add RPC calls.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>