json-parser: Output the content of invalid keyword

When input some invalid word 'unknowcmd' through QMP port, qemu outputs
this error message:
  "parse error: invalid keyword `%s'"

This patch makes qemu output the content of invalid keyword, like:

  "parse error: invalid keyword `unknowcmd'"

Signed-off-by: Amos Kong <akong@redhat.com>
Acked-by: Richard Henderson <rth@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
