qemu: change qemuFindAgentConfig return type
    
    While this is no functional change, whole channel definition is
    going to be needed very soon. Moreover, while touching this obey
    const correctness rule in qemuAgentOpen() - so far it was passed
    regular pointer to channel config even though the function is
    expected to not change pointee at all. Pass const pointer
    instead.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>