A couple of minor memory size option related nits:
- use common name 'memsize' (instead of 'max-size' or just 'size')
- bhyve: update usage with memsize unit suffix, drop legacy "MB" unit
- bhyveload: update usage with memsize unit suffix
- bhyve(8): document default size
- bhyveload(8): use memsize formatting like it's done in bhyve(8)