qemu_monitor: introduce new function to get QOM path
    
    The search is done recursively only through QOM object that has a type
    prefixed with "child<" as this indicate that the QOM is a parent for
    other QOM objects.
    
    The usage is that you give known device name with starting path where to
    search.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>