conf: Parse and format virDomainChrSerialTargetModel
    
    This information will be used to select, and store in the guest
    configuration in order to guarantee ABI stability, the concrete
    (hypervisor-specific) model for serial devices.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>