Use a hash table for storing MCS labels

Use a hash table for storing MCS labels

Instead of using an O(n) efficiency linked list for storing
MCS labels, use a hash table. Instead of having the list
be global, put it in the SELinux driver private data struct
to ensure uniqueness across different instances of the driver.
This also ensures thread safety when multiple hypervisor
drivers are used in the same libvirtd process

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>