ui: reuse ResourceStores default handling for guest description

ui: reuse ResourceStores default handling for guest description

First, there was an additional space between "'" and ")" which lead
me to this.
I could never trigger the case where vmname is undefined, and saw
that we have a almost same default handling in data/ResourceStore.js

So, just use that one instead here - it misses the single-quotes
inside the parentheses, but as there are already said parentheses
this is visually still very OK, IMO.

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