virt: Avoid timeout log spam due to channel timeouts

Currently vmchannels.Listener is logging every timeout events every
30 seconds (based on guest_agent_timeout config value):
vmChannels::91::vds::(_handle_timeouts) Timeout on fileno 106.

This patch changes this behaviour to keep track if a timeout has
been logged before and in that case wouldn't log it again until
a new event (like incoming data) has been received.

Change-Id: I82e868ce7c0ab28a9f7983887a097f760c3d0f69
Bug-Url: https://bugzilla.redhat.com/1096312
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27627
Reviewed-by: Dan Kenigsberg <danken@redhat.com>