vfio/migration: Refactor vfio_save_block() to return saved data size

vfio/migration: Refactor vfio_save_block() to return saved data size

Refactor vfio_save_block() to return the size of saved data on success
and -errno on error.

This will be used in next patch to implement VFIO migration pre-copy
support.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>