bhyvectl: generate usage from options table

The usage text had fallen out of sync with the actually available
options. Rather than keep them in sync by hand, just generate usage from
the available options.

Signed-off-by: Rob Norris <robn@despairlabs.com>

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44641