🌱 Add default registry in kubevirt tilt file

[Karthik-K-N]

What this PR does / why we need it:

If we blindly follow the Developing Cluster API with Tilt in CAPI book for Kubevirt provider references and use the given tilt file content, we hit the following error

ERROR: Traceback (most recent call last):
  /root/cluster-api/Tiltfile:46:13: in <toplevel>
Error in fail: default_registry is required when not using a local registry, please add it to your tilt-settings.yaml/json

This PR would add the default_registry key into the tilt file.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

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[Karthik-K-N]

/cc @johananl