migration: vmsd errp handlers: return bool

migration: vmsd errp handlers: return bool

No code actually depend on specific errno values returned by
vmstate_load_state. The only use of it is to check for success,
and sometimes inject numeric error values into error messages
in migration code. The latter is not a stopper for gradual
conversion to "errp + bool return value" APIs.

Big analysis of vmstate_load_state() callers, showing that
specific errno values are not actually used, is done by Peter
here:

https://lore.kernel.org/qemu-devel/aQDdRn8t0B8oE3gf@x1.local/

Converting of vmstate_load_state() itself will follow in
another series.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Link: https://lore.kernel.org/r/20251028170926.77219-2-vsementsov@yandex-team.ru
Signed-off-by: Peter Xu <peterx@redhat.com>