qemu: Introduce qemuDomainPrepareDiskSource
    
    Introduce a function to setup any TLS needs for a disk source.
    
    If there's a configuration or other error setting up the disk source
    for TLS, then cause the domain startup to fail.
    
    For VxHS, follow the chardevTLS model where if the src->haveTLS hasn't
    been configured, then take the system/global cfg->haveTLS setting for
    the storage source *and* mark that we've done so via the tlsFromConfig
    setting in storage source.
    
    Next, if we are using TLS, then generate an alias into a virStorageSource
    'tlsAlias' field that will be used to create the TLS object and added to
    the disk object in order to link the two together for QEMU.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>