sim,base-stats: Fix leading "." bug when obtaining requestors

When requestor id is requested, it is stripped of the System name via
the `stripSystemName` function in `system.cc`. However, there is a bug
in this code that leaves a leading ".". E.g.:

`system.cpu.mmu.dtb.walker` is stripped to `.cpu.mmu.dtb.walker`.

This patch fixes this issue.

Change-Id: I825cbc60c7f7eaa84c8a0150c30e9f2902cff6cb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41513
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>

src/sim/system.cc

@@ -532,7 +532,7 @@ std::string
 System::stripSystemName(const std::string& requestor_name) const
 {
     if (startswith(requestor_name, name())) {
-        return requestor_name.substr(name().size());
+        return requestor_name.substr(name().size() + 1);
     } else {
         return requestor_name;
     }