vduse: Add suspend

Implement suspend operation for vduse devices, so vhost-vdpa will offer
that backend feature and userspace can effectively suspend the device.

This is a must before get virtqueue indexes (base) for live migration,
since the device could modify them after userland gets them.

This patch does not implement resume, so VMM resets the whole device
to recover from a live migration failure.  Resume optimization can be
implemented on top of these patches, as other vDPA devices have done in
the past.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260707123344.244575-3-eperezma@redhat.com>