disk_backend: add new layered disk (#404) #281

Job	Run time
build mdbook guide	1m 22s
build artifacts [aarch64-linux]	5m 51s
build and check docs [x64-windows]	9m 7s
build artifacts [x64-linux]	6m 43s
build openhcl [aarch64-linux]	7m 51s
build openhcl [x64-linux]	10m 3s
clippy [windows], unit tests [x64-windows]	17m 2s
clippy [linux, macos], unit tests [x64-linux]	12m 7s
clippy [linux-musl, misc nostd], unit tests [x64-linux-musl]	11m 13s
unit tests [aarch64-windows]	10m 5s
build and check docs [x64-linux]	2m 54s
xtask fmt (linux)	2m 24s
test flowey local backend	3m 17s
build artifacts (not for VMM tests) [aarch64-windows]	5m 48s
build artifacts (not for VMM tests) [x64-windows]	5m 43s
build artifacts (for VMM tests) [x64-windows]	16m 17s
build artifacts (for VMM tests) [aarch64-windows]	15m 37s
xtask fmt (windows)	6m 43s
publish openvmm.dev	43s
run vmm-tests [aarch64-windows]	7m 37s
run vmm-tests [x64-windows-amd]	7m 6s
run vmm-tests [x64-windows-intel]	6m 42s
run vmm-tests [x64-linux]	3m 36s
	2h 55m 51s

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