vhost-user-blk-test: pass vhost-user socket fds to QSD

vhost-user-blk-test: pass vhost-user socket fds to QSD

qemu-storage-daemon is launched with the vhost-user listen socket path.
The path is first unlinked before opening the listen socket. This
prevents stale UNIX domain socket files from stopping socket
initialization.

This behavior is undesirable in vhost-user-blk-test and the cause of a
bug:

There is a race condition in vhost-user-blk-test when QEMU launches
before QSD. It connects to the old socket that QSD unlinks and the
vhost-user connection is never serviced, resulting in a hang.

Pass the listen socket fd to QSD to maintain listen socket continuity
and prevent the lost connection.

Fixes: 806952026df41939680abe92b329715b9b4e01cc ("test: new qTest case to test the vhost-user-blk-server")
Cc: Raphael Norwitz <raphael.norwitz@nutanix.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Coiby Xu <coiby.xu@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20211019135655.83067-1-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>