Catch BlockDeviceError when starting instance

This is a forward-port of commit 1149 on the 1.2 branch:
  _GatherAndLinkBlockDevs used to raise the errors.BlockDeviceError
  exception when it failed to create a block device, and with this patch
  set it does so also when it fails to create a symlink to it.

  With this patch we move the call to this function into a pre-existing
  try-except block in the code, and catch the BlockDeviceError exception,
  logging a message and returning a failure state if it happens.

  Reviewed-by: iustinp

The changes are related to the new hypervisor and logging syntax.

Original-Author: ultrotter