qemu: Connect to guest agent after channel hotplug

qemu: Connect to guest agent after channel hotplug

If a user hot-attaches the guest agent channel libvirt would ignore it
until the restart of libvirtd or shutdown/destroy and start of the VM
itself.

This patch adds code that opens or closes the guest agent connection
according to the state of the guest agent channel according to
connect/disconnect events.

To allow opening the channel from the event handler qemuConnectAgent
needed to be exported.