Replace qmp-commands.hx by docs/qmp-commands.txt

Replace qmp-commands.hx by docs/qmp-commands.txt

The only remaining function of qmp-commands.hx is to let us generate
qmp-commands.txt from it.  Replace qmp-commands.hx by qmp-commands.txt.

We intend to move the documentation into the QAPI schema and generate
qapi-commands.txt from it, but not right now.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20160912091913.15831-19-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>