bhyve: improve memory size documentation

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