pckbd: support for commands 0xf0-0xff: Pulse output bit

pckbd: support for commands 0xf0-0xff: Pulse output bit

I have a guest OS which sends the command 0xfd to the keyboard
controller during initialization. To get rid of the message
"qemu: unsupported keyboard cmd=0x%02x\n" I added support for
the pulse output bit commands.

I found the following explanation here:
http://www.win.tue.nl/~aeb/linux/kbd/scancodes-11.html#ss11.3

Command 0xf0-0xff: Pulse output bit
Bits 3-0 of the output port P2 of the keyboard controller may
be pulsed low for approximately 6 µseconds. Bits 3-0 of this
command specify the output port bits to be pulsed. 0: Bit should
be pulsed. 1: Bit should not be modified. The only useful version
of this command is Command 0xfe.
(For MCA, replace 3-0 by 1-0 in the above.)

Command 0xfe: System reset
Pulse bit 0 of the output port P2 of the keyboard controller.
This will reset the CPU.

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>