core: Use debug on register operation.

When register a disk with snapshot,
there is no reason to log an error but instead only use a debug level log.

Change-Id: If302d623ab13217776e0cb6ec49dda9cc59b7b39
Bug-Url: https://bugzilla.redhat.com/1136808
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>