core: Add lock on attach Storage Domain process

Add a lock in the Storage Domain when attaching a Storage Domain to Data
Center.

Change-Id: Icc484b92d5e49cbe8c01f371f057639220e21428
Bug-Url: https://bugzilla.redhat.com/1144845
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>