Rename intermediate qemu-qapi.texi to qemu-qmp-qapi.texi to match its
user qemu-qmp-ref.texi, just like qemu-ga-qapi.texi matches
qemu-ga-ref.texi.

Build the intermediate .texi next to the sources and the final output
in docs/ instead of dumping them into the build root.

Fix version.texi dependencies so that only the targets that actually
need it depend on it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-8-git-send-email-armbru@redhat.com>