bhyve: Address some warnings in bhyverun.c

• Add const and __unused qualifiers where appropriate.
• Localize some global variables.
• Consistently spell vmexit state as "vme" in vmexit handlers, to avoid shadowing the global vm_exit state array.
• Similarly, avoid shadowing "optarg".

MFC after: 2 weeks

(cherry picked from commit 65b8109b4e07ea1fe105ab10539d5baa0a613a14)