xenconsoled: replace gettimeofday with clock_gettime

xenconsoled: replace gettimeofday with clock_gettime

Currently if someone changes the date on the host, xenconsoled may
behave incorrectly due to the use of gettimeofday for the rate control
algorithm. For example I was able to hang the console connected to a
guest just setting the date forward 20 years. To solve the problem we
need to use a time source that doesn't change start point, that is
clock_gettime with CLOCK_MONOTONIC. The only bad side effect is that
it introduces a dependency to librt, but I think is well worth it.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>