vdpa: Add suspend operation

This operation is optional: It it's not implemented, backend feature bit
will not be exposed.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20220810171512.2343333-2-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>