qapi: Add copyright declaration on docs

While our top-level COPYING with its GPLv2+ license applies to
any documentation file that omits explicit instructions, these
days it's better to be a good example of calling out our
intentions.  Correct use of GPL requires the use of a copyright
statement, so I'm adding notice to two QAPI documents, by
attributing these files to the initial authors and major
contributors.  I used:

$ git blame --line-porcelain $file \
  | sed -n 's/^author //p' | sort | uniq -c | sort -rn

to determine authorship of these two files.  qmp-spec.txt blames
entirely to Red Hat (easy, since my contribution falls in that
category); while qapi-code-gen.txt has multiple contributors
representing multiple entities.  But since it was originally
supplied by Michael Roth, the notice I added there copies the
notice he has used in other files.  As there is no intended
change in license from the implicit one previously present from
the top level, I have not bothered to CC other contributors;
if we want to weaken things to something looser (such as LGPL)
so that there is no question that someone re-implementing the
spec is not forced to use GPL, that would be a different commit.

CC: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>