ui: vm: allow to add socket backed serial devices

ui: vm: allow to add socket backed serial devices

We show and can remove serial devices but couldn't add new ones
through the WebUI.
Add a simple component to allow adding serial ports backed by a
socket, which can be especially useful now with xterm.js

Passing through serial devices from /dev isn't possible with this, as
it is normally a root only operation and not that often used.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>