bhyve.8: Improve emulation description of the -s flag

bhyve.8: Improve emulation description of the -s flag

- Set width of the list to the longest key word for readability.
- Separate descriptions of amd_hostbridge and hostbridge emulations.
  Also, wordsmith their descriptions for consistency with other entries.
- Use Cm instead of Li for command modifiers.
- Do not stylize AMD with Li, there's no need to do it.
- Fix a typo in the definition of ahci-hd ("hard drive" instead of
  "hard-drive").

MFC after:	2 weeks

(cherry picked from commit 7014cb2393594ee4a8389c65d507afacf729c041)