Stop polling after reconnecting to libvirtd
    
    When the connection to libvirtd is lost, virt-viewer starts polling for
    libvirtd to come back. The polling mechanism is also used when
    connecting to very old libvirtd which don't support
    virConnectDomainEventDeregisterAny().
    
    Currently, once we could reconnect to libvirtd, virt-viewer will keep
    polling, thus behaving as if the libvirtd connection does not support
    virConnectDomainEventDeregisterAny(). This commit makes sure we stop
    polling once the new libvirtd connection is established.
    
    This has the side-effect of preventing
    https://bugzilla.redhat.com/show_bug.cgi?id=1271519 from occurring with
    recent libvirt as it's caused by some race occurring when using the
    polling code.