serial: Fix double migration data

serial: Fix double migration data

After c9808d60281 we have both an object representing the serial-isa
device and a separate object representing the underlying common serial
uart.  Both of these have vmsd's associated with them and thus the
migration stream ends up with two copies of the migration data - the
serial-isa includes the vmstate of the core serial.   Besides
being wrong, it breaks backwards migration compatibility.

Fix this by removing the dc->vmsd from the core device, so it only
gets migrated by any parent devices including it.
Add a vmstate_serial_mm so that any device that uses serial_mm_init
rather than creating a device still gets migrated.
(That doesn't fix backwards migration for serial_mm_init users,
but does seem to work forwards for ppce500).

Fixes: c9808d60281 ('serial: realize the serial device')
Buglink: https://bugs.launchpad.net/qemu/+bug/1869426
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200330164712.198282-1-dgilbert@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>