Add deprecation notice for bvmconsole and bvmdebug

Now that bhyve(8) supports UART, bvmconsole and bvmdebug are no longer needed.

Mark the '-b' and '-g' flag as deprecated for bhyve(8).

These will be removed in 13.

Reviewed by: jhb, grehan
Approved by: kevans (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27519