bhyve: improve memory size documentation

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)

Reviewed by: wblock, grehan
Approved by: re (kib), wblock, grehan
Differential Revision: https://reviews.freebsd.org/D6952