Add 'platform version' to Instances

[luqmana]

This came up during the Live Migration sync but is applicable beyond that. We need to be able to separate the hardware platform the host is running on (e.g. Gimlet, AMD Rome/Milan) from the platform a Guest VM sees.

We can start with whatever is currently exposed is version 1 but more importantly, we can then build the infrastructure for properly handling instances with different versions (whether during migration or later rack upgrades).

[gjcolombo]

#7211 takes some initial steps toward implementing this. RFD 505 has many more design details.