qemu: agent: Differentiate errors when the agent channel was hotplugged

qemu: agent: Differentiate errors when the agent channel was hotplugged

When the guest agent channel gets hotplugged to a VM, libvirt would
still report that "QEMU guest agent is not configured" rather than
stating that the connection was not established yet.

Currently the code won't be able to connect to the agent after hotplug
but that will change in a later patch.

As the qemuFindAgentConfig() helper is quite helpful in this case move
it to a more usable place and export it.