vfio/migration: Refactor vfio_save_state() return value

vfio/migration: Refactor vfio_save_state() return value

Currently, vfio_save_state() returns 1 regardless of whether there is
more data to send or not. This was done to prevent a fast changing VFIO
device from potentially blocking other devices from sending their data,
as qemu_savevm_state_iterate() serialized devices.

Now that qemu_savevm_state_iterate() no longer serializes devices, there
is no need for that.

Refactor vfio_save_state() to return 0 if more data is available and 1
if no more data is available.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240304105339.20713-3-avihaih@nvidia.com
Signed-off-by: Peter Xu <peterx@redhat.com>