Make validation warnings more clear

It is easy to grep for the warning text and understand why it happens but the warning should make it clear that this is a validation warning, and the issue is unknown property. Maybe something like:

validate vmType vz: ignoring unknown property networks[1]: "Metric"

Notes:
- Unify warning test in wsl2 and vz (vz uses "ignoring" and wsl2 uses "Ignoring")
- This is going to be a big rabbit hole; look at https://github.com/lima-vm/lima/issues/2512 for more thoughts on how it should be refactored
- Maybe wait until @jandubois replaces current FileDeafults with upcoming code from template assembly because that might change things around anyways
- Warning message may not be clear enough - it can help to link to the docs for the invalid thing.

_Originally posted by @nirs in https://github.com/lima-vm/lima/pull/3010#discussion_r1880825054_