cluster - remote block device work

cluster - remote block device work

* xdisk(client) <-> diskiocom(server) now successfully does a full
  message transaction over a VC for xa_open().  Still lots to do.

* Fix I/O request deferrals since the open can occur before the VC is
  fully established.  This fixes a deadlock.

* Fix state->msg tracking in kern_dmsg.c.  Fixes a double-free.

• [DBH] sys/dev/disk/xdisk/xdisk.c
• [DBH] sys/kern/kern_dmsg.c