When a libvirt API is called from the main event loop (which seems to be
common in event-based glib apps), the client IO loop would properly
handle keepalive requests sent by a server but will not actually send
them because the main event loop is blocked with the API. This patch
gets rid of response timer and the thread which is processing keepalive
requests is also responsible for queueing responses for delivery.
(cherry picked from commit bb85f229)

Add virKeepAliveTimeout and virKeepAliveTrigger APIs that can be used to
set poll timeouts and trigger keepalive timer. virKeepAliveTrigger
checks if it is called to early and does nothing in that case.
(cherry picked from commit 28c75382)

The docs for virConnectSetKeepAlive() advertise that this function
should be able to disable keepalives on negative or zero interval time.

This patch removes the check that prohibited this and adds code to
disable keepalives on negative/zero interval.

* src/libvirt.c: virConnectSetKeepAlive(): - remove check for negative
                                             values
* src/rpc/virnetclient.c
* src/rpc/virnetclient.h: - add virNetClientKeepAliveStop() to disable
                            keepalive messages
* src/remote/remote_driver.c: remoteSetKeepAlive(): -add ability to
                                                     disable keepalives
(cherry picked from commit 6446a9e2)

These APIs are used by both client and server RPC layer to handle
processing of keepalive messages.