qapi: Rename generated qmp-marshal.c to qmp-commands.c

All generated .c are named like their .h, except for qmp-marshal.c and
qmp-commands.h.  To add to the confusion, tests-qmp-commands.c falsely
matches generated test-qmp-commands.h.

Get rid of this unnecessary complication.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180211093607.27351-19-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>