vhost: make vhost work queue visible

The vhost work queue allows processing to be done in vhost worker thread
context, which uses the owner process mm.  Access to the vring and guest
memory is typically only possible from vhost worker context so it is
useful to allow work to be queued directly by users.

Currently vhost_net only uses the poll wrappers which do not expose the
work queue functions.  However, for tcm_vhost (vhost_scsi) it will be
necessary to queue custom work.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Zhi Yong Wu <wuzhy@cn.ibm.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>