cluster - xdisk automatic BIO restart

cluster - xdisk automatic BIO restart

* The xdisk driver now detects failed transactions due to failed circuits
  and will restart the BIOs on another circuit or hold onto them until
  connectivity is restored and a new circuit is reforged.

  Automatic restarts only occur if the xa* disk is open()'d (i.e. mounted
  or being accessed by userland).  Kernel disk subsystem probes on attach
  will be failed normally and not stall on lost connectivity.

* subr_diskiocom now reports the correct DMSG error code for failed BIOs
  instead of reporting a kernel error code.