vhost-vdpa: fix backend feature ioctls

vhost-vdpa: fix backend feature ioctls

Commit a34a62eb153c ("vhost-vdpa: support get/set backend features")
introduces two malfunction backend features ioctls:

1) the ioctls was blindly added to vring ioctl instead of vdpa device
   ioctl
2) vhost_set_backend_features() was called when dev mutex has already
   been held which will lead a deadlock

This patch fixes the above issues.

Cc: Eli Cohen <elic@nvidia.com>
Reported-by: Zhu Lingshan <lingshan.zhu@intel.com>
Fixes: a34a62eb153c ("vhost-vdpa: support get/set backend features")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200907104343.31141-1-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>