generator: Add 'Key' parameter type.

generator: Add 'Key' parameter type.

Add a 'Key' parameter type, used for passing sensitive key material
into libguestfs.

Eventually the plan is to mlock() key material into memory.  However
this is very difficult to achieve because the encoded XDR strings
end up in many places.  Therefore users should note that key material
passed to libguestfs might end up in swap.

The only difference between 'Key' and 'String' currently is that
guestfish requests the key from /dev/tty with echoing turned off.