chardev: add file chardev support to chardev-add (qmp)

Add support for file chardevs.  Output file is mandatory,
input file is optional.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

qmp-commands.hx

@@ -2672,13 +2672,19 @@ Arguments:
 - "id": the chardev's ID, must be unique (json-string)
 - "backend": chardev backend type + parameters
 
-Example:
+Examples:
 
 -> { "execute" : "chardev-add",
      "arguments" : { "id" : "foo",
                      "backend" : { "type" : "null", "data" : {} } } }
 <- { "return": {} }
 
+-> { "execute" : "chardev-add",
+     "arguments" : { "id" : "bar",
+                     "backend" : { "type" : "file",
+                                   "data" : { "out" : "/tmp/bar.log" } } } }
+<- { "return": {} }
+
 EQMP
 
     {