Keep vmbus save state backward compatible (#99) #1

Job	Run time
build artifacts [x64-linux]	1s
build and check docs [x64-windows]	1s
build mdbook guide [x64-windows]	1s
build openhcl [aarch64-linux]	1s
build openhcl [x64-linux]	1s
clippy [windows], unit tests [x64-windows]	1s
clippy [linux-musl, misc nostd], unit tests [x64-linux-musl]	1s
clippy [linux, macos], unit tests [x64-linux]	1s
build and check docs [x64-linux]	1s
test flowey local backend	1s
xtask fmt (linux)	1s
build artifacts (not for VMM tests) [aarch64-windows]	1s
build artifacts (for VMM tests) [aarch64-windows]	22m 56s
build artifacts (not for VMM tests) [x64-windows]	1s
build artifacts (for VMM tests) [x64-windows]	23m 1s
build artifacts [aarch64-linux]	1s
run vmm-tests [x64-windows-intel]	0s
run vmm-tests [x64-windows-amd]	0s
run vmm-tests [x64-linux]	0s
xtask fmt (windows)	0s
openvmm checkin gates	0s
	46m 11s

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