standalone: want option to hash contents of boot disk

Incremental work for #1067. Most useful for #1067 when built upon #1068, but technically we could merge this work indepdently.

An example TOML file. The new calc_boot_digest flag requires a boot order specified:

[main]
name = "boot-digest"
cpus = 4
bootrom = "/home/jordan/src/propolis/ovmf/OVMF_CODE.fd"
memory = 1024
calc_boot_digest = true
boot_order = [ "block01" ]

[block_dev.vm_attest_disk]
type = "file"
#path = "/dev/zvol/rdsk/rpool/propolis/vm-attest-img"
path = "/home/jordan/src/propolis/images/vm-instance_staging-a.raw"
read_only = true

[dev.block01]
driver = "pci-nvme"
block_dev = "vm_attest_disk"
pci-path = "0.7.0"


[dev.net0]
driver = "pci-virtio-viona"
vnic = "vnic_prop0"
pci-path = "0.5.0"

An initial pass at just file-backed disks (hashing a local file on disk), then printing out the hash on start is: https://github.com/oxidecomputer/propolis/tree/jhendricks/calc-boot-digest

Start by reviewing the linked calc-boot-digest branch and the TOML example, focusing on how calc_boot_digest and boot_order are represented. The initial scope is file-backed disks: hash the local boot disk contents and print the digest when the VM starts, with the required boot order configuration.