e/desk_wallpaper: Fix setting wallpaper from within Virtual Desk dialog

e/desk_wallpaper: Fix setting wallpaper from within Virtual Desk dialog

Summary:
It seems this was missed when doing the manager/container id removal
back in march.  The param validation was failing and causing the "Set"
button to not work in "Screen > Virtual Desktops > Click to Change
wallpaper".

Test Plan:
1. Open "Screen > Virtual Desktops > Click to Change wallpaper"
2. Click "Set"
  - Expect that a wallpaper dialog be shown
  - Before this patch nothing happens

Reviewers: zmike

Subscribers: cedric, seoz

Differential Revision: https://phab.enlightenment.org/D3298