iscsi: Add exit status checking for virISCSIGetSession

Utilize the exit status parameter for virCommandRunRegex in order to
check the return error from the 'iscsiadm --mode session' command.
Without this enabled, if there are no sessions running then virCommandRun
would have displayed an error such as:

    2016-05-13 15:17:15.165+0000: 10920: error : virCommandWait:2553 :
               internal error: Child process (iscsiadm --mode session)
               unexpected exit status 21: iscsiadm: No active sessions.

It is possible that for certain paths (when probe is true) we only care
whether it's running or not to make certain decisions.  Spitting out
the error for those paths is unnecessary.

If we do have a situation where probe = false and there's an error,
then display the error from iscsiadm if it's there.

src/util/viriscsi.c

@@ -79,24 +79,28 @@ virISCSIGetSession(const char *devpath,
         .session = NULL,
         .devpath = devpath,
     };
+    char *error = NULL;
+    int exitstatus = 0;
 
-    virCommandPtr cmd = virCommandNewArgList(ISCSIADM, "--mode", "session", NULL);
+    virCommandPtr cmd = virCommandNewArgList(ISCSIADM, "--mode",
+                                             "session", NULL);
+    virCommandSetErrorBuffer(cmd, &error);
 
     if (virCommandRunRegex(cmd,
                            1,
                            regexes,
                            vars,
                            virISCSIExtractSession,
-                           &cbdata, NULL, NULL) < 0)
+                           &cbdata, NULL, &exitstatus) < 0)
         goto cleanup;
 
-    if (cbdata.session == NULL && !probe) {
+    if (cbdata.session == NULL && !probe)
         virReportError(VIR_ERR_INTERNAL_ERROR,
-                       "%s", _("cannot find session"));
-        goto cleanup;
-    }
+                       _("cannot find iscsiadm session: %s"),
+                       NULLSTR(error));
 
  cleanup:
+    VIR_FREE(error);
     virCommandFree(cmd);
     return cbdata.session;
 }