vhost-vdpa: stick to -errno error return convention

vhost-vdpa: stick to -errno error return convention

Almost all VhostOps methods in vdpa_ops follow the convention of
returning negated errno on error.

Adjust the few that don't.  To that end, rework vhost_vdpa_add_status to
check if setting of the requested status bits has succeeded and return
the respective error code it hasn't, and propagate the error codes
wherever it's appropriate.

Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
Message-Id: <20211111153354.18807-8-rvkagan@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>