admin: Resolve leaked reference to private data

Running valgrind on a very simplistic program consisting only of
opening and closing admin connection (virAdmConnect{Open,Close}) shows a
leak in remoteAdminPrivNew, because the last reference to privateData is
not decremented, thus the object won't be disposed. This patch unrefs
the privateData object once we closed the active connection to daemon,
making further use of this connection  useless.

==24577==    at 0x4A089C7: calloc (in /usr/lib64/valgrind/vgpreload_***linux.so)
==24577==    by 0x4E8835F: virAllocVar (viralloc.c:560)
==24577==    by 0x4EDFA5C: virObjectNew (virobject.c:193)
==24577==    by 0x4EDFBD4: virObjectLockableNew (virobject.c:219)
==24577==    by 0x4C14DAF: remoteAdminPrivNew (libvirt-admin.c:152)
==24577==    by 0x4C1537E: virAdmConnectOpen (libvirt-admin.c:308)
==24577==    by 0x400BAD: main (listservers.c:39)

==24577== LEAK SUMMARY:
==24577==    definitely lost: 80 bytes in 1 blocks
==24577==    indirectly lost: 840 bytes in 6 blocks
==24577==      possibly lost: 0 bytes in 0 blocks
==24577==    still reachable: 12,179 bytes in 199 blocks
==24577==         suppressed: 0 bytes in 0 blocks

src/libvirt-admin.c

@@ -132,6 +132,7 @@ remoteAdminPrivFree(void *opaque)
     virAdmConnectPtr conn = opaque;
 
     remoteAdminConnectClose(conn);
+    virObjectUnref(conn->privateData);
 }
 
 static remoteAdminPrivPtr