This adds an interface for a backend-specific notion of time, e.g. CPU
cycles since simulator start, and a concrete implementation for the
Bochs backend. This is needed to record CPU idle times (e.g., HLT
instruction), and for target backends capable of more timing-accurate
execution.
This change also modifies the tracing plugin to add the time to all
trace events.
Change-Id: I93ac1d54c07f32b0b8f84f333417741d8e9c8288
e5fe9dd525