qemu: Use secret objects to pass iSCSI passwords
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1425757
    
    The blockdev-add code provides a mechanism to sanely provide user
    and password-secret arguments for iscsi without placing them on the
    command line to be viewable by a 'ps -ef' type command or needing
    to create separate -iscsi devices for each disk/volume found.
    
    So modify the iSCSI command line building to check for the presence
    of the capability in order properly setup and use the domain master
    secret object to encrypt the password in a secret object and alter
    the parameters for the command line to utilize.
    
    Modify the xml2argvtest to exhibit the syntax for both disk and
    hostdev configurations.