more perf_event_open cleanups

This has some cleanups for the perf_event_open call
+ Makes the switch statements all have explicit case statements
  for all the cases we care about (instead of sometimes relying
  on default)
+ Uses rand32() or rand64() over rand() when appropriate
+ Adds a lot more coverage to the create_random_event() function
+ Some other fine tuning

In the end I didn't run lindent on it.  I agree the file could use it,
but I didn't want to mix indentation changes in with functionality
changes.

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Fix divide by zero in random_sysfs_config

> [221186.493638] traps: trinity-child30[375] trap divide error
> ip:40dbae sp:7fff3c1d7ae0 error:0 in trinity[400000+28000]

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>