conf: add VIR_DOMAIN_CHR_SERIAL_TARGET_TYPE_SCLP
    
    Introduce specific a target types with two models for the console
    devices (sclp and sclplm) used in s390 and s390x guests, so isa-serial
    is no more used for them.
    
    This makes <serial> usable on s390 and s390x guests, with at most only
    a single sclpconsole and one sclplmconsole devices usable in a single
    guest (due to limitations in QEMU, which will enforce already at
    runtime).
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1449265
    
    Signed-off-by: Pino Toscano <ptoscano@redhat.com>
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>