Higher throughput via indirect descriptors

• blkback: add 'force_close' to more forcibly tear down the device
• blkback: make 'destroy' idempotent
• blkback: measure ring utilisation; segments per request; total
  requests and responses (ok and error)
• blkback: support indirect descriptors (i.e. large block sizes)
• blkfront: if the 'connect' string is at all ambiguous, fail rather
  than risk using the wrong disk
• blkfront: use indirect segments if available