Add an optional virTristateBool haveTLS to virStorageSource to
manage whether a storage source will be using TLS.

Sample XML for a VxHS disk:

<disk type='network' device='disk'>
  <driver name='qemu' type='raw' cache='none'/>
  <source protocol='vxhs' name='eb90327c-8302-4725-9e1b-4e85ed4dc251' tls='yes'>
    <host name='192.168.0.1' port='9999'/>
  </source>
  <target dev='vda' bus='virtio'/>
</disk>

Additionally add a tlsFromConfig boolean to control whether the TLS
setting was due to domain configuration or qemu.conf global setting
in order to decide whether to Format the haveTLS setting for either
a live or saved domain configuration file.

Update the qemuxml2xmltest in order to add a test to show the proper
parsing.

Also update the docs to describe the tls attribute.
Signed-off-by: NAshish Mittal <Ashish.Mittal@veritas.com>
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>