acrn-dm: fix corner cases in acrn_parse_cpu_affinity()

- re-arange the code to make static code analysis tool happy.

- If no valid conversion could be performed, a zero value is returned
  (0L) from strtol(), so add a sanity check "isdigit(cp[0])" to ensure
  that it won't unexpectedly parse CPU 0 if the string starts or ends
  with the valid delimiters ',' or '-', for example:

  -- cpu_affinity 1,
  -- cpu_affinity ,1

Tracked-On: #4616
Signed-off-by: Zide Chen <zide.chen@intel.com>

devicemodel/core/vmmapi.c

@@ -132,17 +132,17 @@ int acrn_parse_cpu_affinity(char *opt)
 		return -1;
 	}
 
-	while (cp) {
+	/* white spaces within the commane line are invalid */
+	while (cp && isdigit(cp[0])) {
 		str = strpbrk(cp, ",-");
 
 		/* no more entries delimited by ',' or '-' */
 		if (!str) {
 			if (!dm_strtoi(cp, NULL, 10, &pcpu_id)) {
 				add_one_pcpu(pcpu_id);
+			}
 			break;
-			}
+		} else {
-		}
-
 			if (*str == ',') {
 				/* after this, 'cp' points to the character after ',' */
 				str = strsep(&cp, ",");
@@ -174,6 +174,7 @@ int acrn_parse_cpu_affinity(char *opt)
 				str = strsep(&cp, ",");
 			}
 		}
+	}
 
 	return 0;
 }