vhost: move the backend feature bits to vhost_types.h

We should store feature bits in vhost_types.h as what has been done
for e.g VHOST_F_LOG_ALL.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-2-gdawar@xilinx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>