util: Report error if vhost-scsi device file cannot be found

https://bugzilla.redhat.com/show_bug.cgi?id=1523564

If the vhost-scsi device file cannot be found, the generic error

    "error: An error occurred, but the cause is unknown"

is returned.  Let's add a real error message to make it clear
why the failure occurred.

src/util/virscsivhost.c

@@ -86,8 +86,12 @@ VIR_ONCE_GLOBAL_INIT(virSCSIVHost)
 int
 virSCSIVHostOpenVhostSCSI(int *vhostfd)
 {
-    if (!virFileExists(VHOST_SCSI_DEVICE))
+    if (!virFileExists(VHOST_SCSI_DEVICE)) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                       _("vhost-scsi device file '%s' cannot be found"),
+                       VHOST_SCSI_DEVICE);
         return -1;
+    }
 
     *vhostfd = open(VHOST_SCSI_DEVICE, O_RDWR);