This reverts commit adb29c02.

The commit broke -device vhost-user-blk-pci because the
vhost_dev_prepare_inflight() function it introduced segfaults in
vhost_dev_set_features() when attempting to access struct vhost_dev's
vdev pointer before it has been assigned.

To reproduce the segfault simply launch a vhost-user-blk device with the
contrib vhost-user-blk device backend:

  $ build/contrib/vhost-user-blk/vhost-user-blk -s /tmp/vhost-user-blk.sock -r -b /var/tmp/foo.img
  $ build/qemu-system-x86_64 \
        -device vhost-user-blk-pci,id=drv0,chardev=char1,addr=4.0 \
        -object memory-backend-memfd,id=mem,size=1G,share=on \
        -M memory-backend=mem,accel=kvm \
        -chardev socket,id=char1,path=/tmp/vhost-user-blk.sock
  Segmentation fault (core dumped)

Cc: Jin Yu <jin.yu@intel.com>
Cc: Raphael Norwitz <raphael.norwitz@nutanix.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20201102165709.232180-1-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>