backends/vhost-user: remove the ioeventfd check

backends/vhost-user: remove the ioeventfd check

While ioeventfds are needed for good performance with KVM guests it
should not be a gating requirement. We can run vhost-user backends using
simulated ioeventfds or inband signalling.

With this change I can run:

  $QEMU $OPTS \
    -display gtk,gl=on \
    -device vhost-user-gpu-pci,chardev=vhgpu \
    -chardev socket,id=vhgpu,path=vhgpu.sock

with:

  ./contrib/vhost-user-gpu/vhost-user-gpu \
    -s vhgpu.sock \
    -v

and at least see things start-up - although the display gets rotated by
180 degrees. Once lightdm takes over we never make it to the login
prompt and just get a blank screen.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20221202132231.1048669-1-alex.bennee@linaro.org>

Message-Id: <20230130124728.175610-1-alex.bennee@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>