pseries: Update SLOF firmware image

Various fixes in this update, the full list is:

  > qemu-bootlist: Take the "-boot strict=off" setting properly into account
  > virtio-scsi: initialize vring avail queue buffers
  > virtio: Remove global variables in block and 9p driver
  > Remove superfluous checkpoints in tree.fs
  > Provide "write" function in the disk-label package
  > virtio: Implement block write support
  > scsi: Add SCSI block write support
  > deblocker: Add a 'write' function
  > virtio-scsi: Fix descriptor order for SCSI WRITE commands
  > board-qemu: Add a possibility to use hvterm input instead of USB keyboard
  > Do not try to use virtio-gpu in VGA mode
  > virtio: Fix stack comment of virtio-blk-read
  > envvar: Do not read default values for /options from the NVRAM anymore
  > envvar: Set properties in /options during "(set-defaults)"

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

pc-bios/README

@@ -17,7 +17,7 @@
 - SLOF (Slimline Open Firmware) is a free IEEE 1275 Open Firmware
   implementation for certain IBM POWER hardware.  The sources are at
   https://github.com/aik/SLOF, and the image currently in qemu is
-  built from git tag qemu-slof-20161019.
+  built from git tag qemu-slof-20170303.
 
 - sgabios (the Serial Graphics Adapter option ROM) provides a means for
   legacy x86 software to communicate with an attached serial console as

SLOF @ 66d250ef

-Subproject commit efd65f49929d7db775b26066d538c8120ae3db94
+Subproject commit 66d250ef0fd06bb88b7399b9563b5008201f2d63