This query copies the kernel command line into the message buffer. It
was previously stubbed out to return empty, this commit makes it reflect
the arguments specified with `-append`.

I observed the following peculiarities on my Pi 3B+:
- If the buffer is shorter than the string, the response header gives
  the full length, but no data is actually copied.
- No NUL terminator is added: even if the buffer is long enough to fit
  one, the buffer's original contents are preserved past the string's
  end.
- The VC firmware adds the following extra parameters beside the
  user-supplied ones (via /boot/cmdline.txt): `video`, `vc_mem.mem_base`
  and `vc_mem.mem_size`. This is currently not implemented in qemu.

Signed-off-by: Daniel Bertalan <dani@danielbertalan.dev>
Message-id: 20230425103250.56653-1-dani@danielbertalan.dev
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: added comment about NUL and short-buffer behaviour]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>