chardev: ensure qemu_chr_parse_compat reports missing driver error

If no valid char driver was identified the qemu_chr_parse_compat method
was silent, leaving callers no clue what failed.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190211182442.8542-8-berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

chardev/char.c

@@ -490,6 +490,8 @@ QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename,
         return opts;
     }
 
+    error_report("'%s' is not a valid char driver", filename);
+
 fail:
     qemu_opts_del(opts);
     return NULL;

tests/test-char.c

@@ -932,9 +932,10 @@ static void char_null_test(void)
 static void char_invalid_test(void)
 {
     Chardev *chr;
-
+    g_setenv("QTEST_SILENT_ERRORS", "1", 1);
     chr = qemu_chr_new("label-invalid", "invalid");
     g_assert_null(chr);
+    g_unsetenv("QTEST_SILENT_ERRORS");
 }
 
 static int chardev_change(void *opaque)