qemu: Support usb-serial and pci-serial on pSeries
    
    The existing implementation set the address type for all serial
    devices to spapr-vio, which made it impossible to use other devices
    such as usb-serial and pci-serial; moreover, some decisions were
    made based on the address type rather than the device type.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1512934
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>