core: Add a lock when detaching Storage from Data Center

Add a lock in the Storage Domain when detaching a Storage Domain from Data Center.

Bug-Url: https://bugzilla.redhat.com/1144845
Change-Id: I42c6be592ad6d71579ce0aa20926856cbca23d44
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>