add timestamp to error_report()

add timestamp to error_report()

[Issue]
When we offer a customer support service and a problem happens
in a customer's system, we try to understand the problem by
comparing what the customer reports with message logs of the
customer's system.

In this case, we often need to know when the problem happens.

But, currently, there is no timestamp in qemu's error messages.
Therefore, we may not be able to understand the problem based on
error messages.

[Solution]
Add a timestamp to qemu's error message logged by
error_report() with g_time_val_to_iso8601().

Signed-off-by: Seiji Aguchi <seiji.aguchi@hds.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>