collector/perf: Run perf as root if the target is rooted

If you want to collect events by event id (eg. in simpleperf, "rNNN"
with NNN a number), you must run it as root.  Otherwise, simpleperf
fails with a SIGABRT.

Run simpleperf as root by default if the target is rooted to avoid
this.

devlib/collector/perf.py

@@ -141,7 +141,7 @@ class PerfCollector(CollectorBase):
 
     def start(self):
         for command in self.commands:
-            self.target.kick_off(command)
+            self.target.kick_off(command, as_root=self.target.is_rooted)
 
     def stop(self):
         self.target.killall(self.perf_type, signal='SIGINT',