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)