device: cleanup input device code
    
    The current code was a little bit odd.  At first we've removed all
    possible implicit input devices from domain definition to add them later
    back if there was any graphics device defined while parsing XML
    description.  That's not all, while formating domain definition to XML
    description we at first ignore any input devices with bus different to
    USB and VIRTIO and few lines later we add implicit input devices to XML.
    
    This seems to me as a lot of code for nothing.  This patch may look
    to be more complicated than original approach, but this is a preferred
    way to modify/add driver specific stuff only in those drivers and not
    deal with them in common parsing/formating functions.
    
    The update is to add those implicit input devices into config XML to
    follow the real HW configuration visible by guest OS.
    
    There was also inconsistence between our behavior and QEMU's in the way,
    that in QEMU there is no way how to disable those implicit input devices
    for x86 architecture and they are available always, even without graphics
    device.  This applies also to XEN hypervisor.  VZ driver already does its
    part by putting correct implicit devices into live XML.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>