Ensure new creating instances are shown consistently during creation

[edlerd]

Done

• ensure new creating instances are shown consistently. previously instances would shortly vanish from the instance list after creation and then reappear.

Fixes #549

QA

1. Run the LXD-UI:
   • On the demo server via the link posted by @webteam-app below. This is only available for PRs created by collaborators of the repo. Ask @lorumic or @edlerd for access.
   • With a local copy of this branch, run as described here.
2. Perform the following QA steps:
   • go to a project with few or no instances. Create a new instance hit create and start. Ensure the instance list does not get shorter until the instance is started.

[webteam-app]

Demo starting at https://lxd-ui-557.demos.haus