add JSON output format to cpuid-gen

[gjcolombo]

Add a JSON output format to cpuid-gen, which prints output suitable for inserting into an instance spec that can be fed to propolis-cli.

This is useful for ad hoc testing of hypervisor enlightenments. Enlightenments are exposed through special CPUID leaves in the hypervisor leaf range (0x4000_0000 to 0x4000_FFFF). To expose enlightenments, Propolis thus needs to pass the correct CPUID values to bhyve. However, if Propolis starts setting CPUID values for the hypervisor leaves, it will also need to set them for the standard and extended leaves. This implies that, to enable enlightenments, Propolis's instance-ensure API (or propolis-standalone's VM setup logic) will need a complete set of standard/extended CPUID values to apply. For propolis-server, the easiest way to do this is to use propolis-cli new --spec. The goal of this change is to make it easy to get a CPUID JSON blob to paste into this command's input spec.

While dorking around with this change, I discovered that the propolis-cli new --spec switch was not properly marked as exclusive with the --config-toml switch. Fix that up as well.

Finally, if propolis-server fails to convert an API spec to its internal instance spec type, and the resulting error has a source, print both the error and the source. This allows the display of messages like "leaf 7 is specified both with and without a subleaf" when appropriate.

Tested by booting some ad hoc propolis-server VMs using propolis-cli and the new output from cpuid-gen.

[hawkw]

i had a few very minor suggestions you may find useful, but this looks good to me!