docs: add a note that spice channel is usable only with spice graphics
    
    To prevent a confusion about missing chardev argument in qemu
    command line add a note about that behavior into documentation.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1129198
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>