vdpa: Track device suspended state

Set vdpa device suspended state on successful suspend. Clear it on
successful resume and reset.

The state will be locked by the vhost_vdpa mutex. The mutex is taken
during suspend, resume and reset in vhost_vdpa_unlocked_ioctl. The
exception is vhost_vdpa_open which does a device reset but that should
be safe because it can only happen before the other ops.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Suggested-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20231225134210.151540-2-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>