bhyve: Fix some warnings in the ps2 emulation code

• Include headers containing prototypes for exported functions.
• Initialize all fields of the extended translation table.
• Qualify an unexported translation table as static.
• Fix error handling for a read(2).
• Fix some style bugs.

No functional change intended.

MFC after: 1 week

(cherry picked from commit 33dfef5cf3df138026491ac5725d55a23ae410f7)