ui: fix regression handling bare 'websocket' option to -vnc

The -vnc argument is documented as accepting two syntaxes for
the 'websocket' option, either a bare option name, or a port
number. If using the bare option name, it is supposed to apply
the display number as an offset to base port 5700. e.g.

  -vnc localhost:3,websocket

should listen on port 5703, however, this was broken in 2.3.0 since

  commit 4db14629
  Author: Gerd Hoffmann <kraxel@redhat.com>
  Date:   Tue Sep 16 12:33:03 2014 +0200

    vnc: switch to QemuOpts, allow multiple servers

instead qemu tries to listen on port "on" which gets looked up in
/etc/services and fails.

Fixes bug: #1455912

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170203120649.15637-2-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>