locking: restrict sockets to mode 0600

locking: restrict sockets to mode 0600

The virtlockd daemon's only intended client is the libvirtd daemon. As
such it should never allow clients from other user accounts to connect.
The code already enforces this and drops clients from other UIDs, but
we can get earlier (and thus stronger) protection against DoS by setting
the socket permissions to 0600

Fixes CVE-2019-10132

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>