compile with vhost-vsock support on osx

[b s]

compiling latest (3.1.0) on osx 10.14.3 with --enable-vhost-vsock and target = x86_64-softmmu results in compile errors:

Undefined symbols for architecture x86_64:
  "_vhost_dev_cleanup", referenced from:
      _vhost_vsock_device_realize in vhost-vsock.o
      _vhost_vsock_device_unrealize in vhost-vsock.o
  "_vhost_dev_disable_notifiers", referenced from:
      _vhost_vsock_set_status in vhost-vsock.o
  "_vhost_dev_enable_notifiers", referenced from:
      _vhost_vsock_set_status in vhost-vsock.o
  "_vhost_dev_init", referenced from:
      _vhost_vsock_device_realize in vhost-vsock.o
  "_vhost_dev_start", referenced from:
      _vhost_vsock_set_status in vhost-vsock.o
  "_vhost_dev_stop", referenced from:
      _vhost_vsock_set_status in vhost-vsock.o
  "_vhost_virtqueue_mask", referenced from:
      _vhost_vsock_set_status in vhost-vsock.o
      _vhost_vsock_guest_notifier_mask in vhost-vsock.o
     (maybe you meant: _cryptodev_vhost_virtqueue_mask)
  "_vhost_virtqueue_pending", referenced from:
      _vhost_vsock_guest_notifier_pending in vhost-vsock.o
     (maybe you meant: _cryptodev_vhost_virtqueue_pending)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[Stefan Hajnoczi (stefanha)]

vhost devices are not available on macOS hosts, they are a Linux feature.