qtest: Don't segfault with invalid -qtest option

This prints an error message, instead of core dump, when "-qtest"
option value is invalid, e.g.:

    $ ./x86_64-softmmu/qemu-system-x86_64 -qtest unknown
        qemu-system-x86_64: Failed to initialize device for qtest:
        "unknown"
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

include/sysemu/qtest.h

@@ -15,6 +15,7 @@
 #define QTEST_H
 
 #include "qemu-common.h"
+#include "qapi/error.h"
 
 extern bool qtest_allowed;
 
@@ -26,7 +27,7 @@ static inline bool qtest_enabled(void)
 bool qtest_driver(void);
 
 int qtest_init_accel(void);
-void qtest_init(const char *qtest_chrdev, const char *qtest_log);
+void qtest_init(const char *qtest_chrdev, const char *qtest_log, Error **errp);
 
 static inline int qtest_available(void)
 {

qtest.c

@@ -507,12 +507,18 @@ int qtest_init_accel(void)
     return 0;
 }
 
-void qtest_init(const char *qtest_chrdev, const char *qtest_log)
+void qtest_init(const char *qtest_chrdev, const char *qtest_log, Error **errp)
 {
     CharDriverState *chr;
 
     chr = qemu_chr_new("qtest", qtest_chrdev, NULL);
 
+    if (chr == NULL) {
+        error_setg(errp, "Failed to initialize device for qtest: \"%s\"",
+                   qtest_chrdev);
+        return;
+    }
+
     qemu_chr_add_handlers(chr, qtest_can_read, qtest_read, qtest_event, chr);
     qemu_chr_fe_set_echo(chr, true);
 

vl.c

@@ -4078,7 +4078,13 @@ int main(int argc, char **argv, char **envp)
     configure_accelerator();
 
     if (qtest_chrdev) {
-        qtest_init(qtest_chrdev, qtest_log);
+        Error *local_err = NULL;
+        qtest_init(qtest_chrdev, qtest_log, &local_err);
+        if (local_err) {
+            error_report("%s", error_get_pretty(local_err));
+            error_free(local_err);
+            exit(1);
+        }
     }
 
     machine_opts = qemu_get_machine_opts();