qemu: Remove redundancy in qemuBuildSerialChrDeviceStr()
    
    Instead duplicating the capability check for each possible target
    model, introduce a small helper that matches the target model with
    the corresponding capability and collapse all existing checks into
    a single one.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>