Simplifies the schema and the code.

QMP command

    { "execute" : "chardev-add",
      "arguments" : { "id" : "ser0",
                      "backend" : { "type" : "port",
                                    "data" : { "type": "serial",
                                               "device":"/dev/ttyS0"} } } }

becomes

    { "execute" : "chardev-add",
      "arguments" : { "id" : "ser0",
                      "backend" : { "type" : "serial",
                                    "data" : { "device":"/dev/ttyS0"} } } }

Bonus: nicer error messages.  "unknown chardev port (1)" becomes
"character device backend type 'parallel' not supported".
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Message-id: 1360767256-610-3-git-send-email-armbru@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>