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

(cherry picked from commit b9fa1500cb2265b95927e19b9d2119ca26d65be3)