Fix up dangling references to qmp-commands.* in comment and doc

Fix up dangling references to qmp-commands.* in comment and doc

Fix up the reference to qmp-commands.hx in qmp.c.  Missed in commit
5032a16d1d.

Fix up the reference to qmp-commands.txt in
docs/xen-save-devices-state.txt.  Missed in commit 4d8bb958fa.

Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20180211093607[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Michael Roth <[email protected]>
Signed-off-by: Eric Blake <[email protected]>