security: plug regression introduced in disk probe logic

security: plug regression introduced in disk probe logic

Regression introduced in commit d6623003 (v0.8.8) - using the
wrong sizeof operand meant that security manager private data
was overlaying the allowDiskFormatProbing member of struct
_virSecurityManager.  This reopens disk probing, which was
supposed to be prevented by the solution to CVE-2010-2238.

* src/security/security_manager.c
(virSecurityManagerGetPrivateData): Use correct offset.