cpus: check cpu->running in cpu_get_icount_raw()

cpus: check cpu->running in cpu_get_icount_raw()

The lifetime of current_cpu is now the lifetime of the vCPU thread.
However get_icount_raw() can apply a fudge factor if called while code
is running to take into account the current executed instruction
count.

To ensure this is always the case we also check cpu->running.

Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>