bhyve: ps2 implement command 0xf6

Implement PS2 Keyboard command 0xf6, which is "SET DEFAULTS". This is
the same as 0xf5 (DISABLE KEYBOARD), but without disabling the keyboard
(since that resets all the defaults as a side effect). Normally, we
clear the fifo when we re-enable the keyboard. However, since this
leaves the keyboard enabled, clear the fifo as part of this command and
send an ack.

Linux's keyboard driver sends this command on reboot. Other commands
enable / reset the kebyoard, so it doesn't matter too much this isn't
implemented for booting, eg ubuntu.

Sponsored by: Netflix
Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D42384