keepalive: Add ability to disable keepalive messages

keepalive: Add ability to disable keepalive messages

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