bhyve/usage: memory size is not in MB

For backward compatibility, the memory size will be interpreted in MB if
it's smaller than1 MB and has no suffix. Nowadays, the -m switch accepts
more than just MB. Respect it in the usage message.

Differential Revision: https://reviews.freebsd.org/D34506
Reviewed by: grehan
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 1 month

(cherry picked from commit afd4f7fa25324608c59a56f38cf4c624522c99fc)