vhost-user.rst: Migrating back-end-internal state

For vhost-user devices, qemu can migrate the virtio state, but not the
back-end's internal state.  To do so, we need to be able to transfer
this internal state between front-end (qemu) and back-end.

At this point, this new feature is added for the purpose of virtio-fs
migration.  Because virtiofsd's internal state will not be too large, we
believe it is best to transfer it as a single binary blob after the
streaming phase.

These are the additions to the protocol:
- New vhost-user protocol feature VHOST_USER_PROTOCOL_F_DEVICE_STATE
- SET_DEVICE_STATE_FD function: Front-end and back-end negotiate a file
  descriptor over which to transfer the state.
- CHECK_DEVICE_STATE: After the state has been transferred through the
  file descriptor, the front-end invokes this function to verify
  success.  There is no in-band way (through the file descriptor) to
  indicate failure, so we need to check explicitly.

Once the transfer FD has been established via SET_DEVICE_STATE_FD
(which includes establishing the direction of transfer and migration
phase), the sending side writes its data into it, and the reading side
reads it until it sees an EOF.  Then, the front-end will check for
success via CHECK_DEVICE_STATE, which on the destination side includes
checking for integrity (i.e. errors during deserialization).

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20231016134243.68248-5-hreitz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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