colo: check chardev can switch context

COLO uses a worker context (iothread) to drive the chardev. All
backends are not able to switch the context, let's report an error in
this case.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181205203737.9011-6-marcandre.lureau@redhat.com>
Reviewed-by: NLi Zhijian <lizhiian@cn.fujitsu.com>
Reviewed-by: NZhang Chen <zhangckid@gmail.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

net/colo-compare.c

@@ -957,6 +957,12 @@ static int find_and_check_chardev(Chardev **chr,
         return 1;
     }
 
+    if (!qemu_chr_has_feature(*chr, QEMU_CHAR_FEATURE_GCONTEXT)) {
+        error_setg(errp, "chardev \"%s\" cannot switch context",
+                   chr_name);
+        return 1;
+    }
+
     return 0;
 }