qemu: change qemuFindAgentConfig return type

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>