bhyve: Address some warnings in bhyverun.c

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