vhost-user-scsi: avoid unlink(NULL) with fd passing

vhost-user-scsi: avoid unlink(NULL) with fd passing

Commit 747421e949fc1eb3ba66b5fcccdb7ba051918241 ("Implements Backend
Program conventions for vhost-user-scsi") introduced fd-passing support
as part of implementing the vhost-user backend program conventions.

When fd passing is used the UNIX domain socket path is NULL and we must
not call unlink(2).

The unlink(2) call is necessary when the listen socket, lsock, was
created successfully since that means the UNIX domain socket is visible
in the file system.

Fixes: Coverity CID 1488353
Fixes: 747421e949fc1eb3ba66b5fcccdb7ba051918241 ("Implements Backend Program conventions for vhost-user-scsi")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20220516155701.1789638-1-stefanha@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>