consider more flexible VMM reservoir configuration for dev setups #3874
Description
Currently one can configure the size of the VMM reservoir via the sled agent config.toml using a percentage. This is a reasonable starting point for a machine with a lot of memory, such as gimlets. For smaller machines running omicron (like in development), having a bit more flexibility here could be nice -- for example, being able to specify a particular size in MiB instead of a percentage of DRAM. This new configuration option should be mutually exclusive with the existing vmm_reservoir_percentage option.