tests: qemu: Don't leak security manager object

==2064442== 200 (88 direct, 112 indirect) bytes in 1 blocks are definitely lost in loss record 54 of 73
==2064442==    at 0x4C2E0F0: calloc (vg_replace_malloc.c:711)
==2064442==    by 0x18E75B80: virAllocVar (viralloc.c:560)
==2064442==    by 0x18EC43B0: virObjectNew (virobject.c:193)
==2064442==    by 0x18EC476E: virObjectLockableNew (virobject.c:219)
==2064442==    by 0x1906BC73: virSecurityManagerNewDriver (security_manager.c:93)
==2064442==    by 0x1906C076: virSecurityManagerNewStack (security_manager.c:115)
==2064442==    by 0x43CC39: qemuTestDriverInit (testutilsqemu.c:548)
==2064442==    by 0x4337ED: mymain (qemumonitorjsontest.c:2440)
==2064442==    by 0x43BABE: virTestMain (testutils.c:982)
==2064442==    by 0x43A490: main (qemumonitorjsontest.c:2558)

tests/testutilsqemu.c

@@ -475,6 +475,7 @@ void qemuTestDriverFree(virQEMUDriver *driver)
     virObjectUnref(driver->xmlopt);
     virObjectUnref(driver->caps);
     virObjectUnref(driver->config);
+    virObjectUnref(driver->securityManager);
 }
 
 int qemuTestCapsCacheInsert(virQEMUCapsCachePtr cache, const char *binary,