Expose option to configure size of microvm root volumes

Users would like to create their microvm nodes with larger attached volumes.

The flintlock api exposes an option to resize the root volume of a microvm, assuming this is the correct dial to adjust, let's expose this at the capmvm level.

Start by tracing the capmvm API path that creates microVM nodes and compare it with the flintlock API's root-volume resize option. Identify the request and configuration types involved, then add the option so callers can set the root volume size and verify that the value reaches flintlock.

Written by the indexing model from the issue text.