Fix up the reference to qmp-commands.hx in qmp.c.  Missed in commit
5032a16d.

Fix up the reference to qmp-commands.txt in
docs/xen-save-devices-state.txt.  Missed in commit 4d8bb958.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180211093607.27351-29-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>