Add a boot device field to Instance, forward along as appropriate #38319

Job	Run time
build-docs	8m 52s
check-features	11m 13s
check-omicron-deployment (ubuntu-22.04)	8m 39s
check-style	34s
clippy-lint	10m 14s
check-omicron-deployment (macos-14)	9m 46s
	49m 18s

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