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>

backends/vhost-user.c

@@ -20,12 +20,6 @@
 #include "io/channel-command.h"
 #include "hw/virtio/virtio-bus.h"
 
-static bool
-ioeventfd_enabled(void)
-{
-    return kvm_enabled() && kvm_eventfds_enabled();
-}
-
 int
 vhost_user_backend_dev_init(VhostUserBackend *b, VirtIODevice *vdev,
                             unsigned nvqs, Error **errp)
@@ -34,11 +28,6 @@ vhost_user_backend_dev_init(VhostUserBackend *b, VirtIODevice *vdev,
 
     assert(!b->vdev && vdev);
 
-    if (!ioeventfd_enabled()) {
-        error_setg(errp, "vhost initialization failed: requires kvm");
-        return -1;
-    }
-
     if (!vhost_user_init(&b->vhost_user, &b->chr, errp)) {
         return -1;
     }