Merge remote-tracking branch 'remotes/jnsnow/tags/ide-pull-request' into staging

# gpg: Signature made Mon 18 Jul 2016 23:53:15 BST
# gpg:                using RSA key 0x7DEF8106AAFC390E
# gpg: Good signature from "John Snow (John Huston) <[email protected]>"
# Primary key fingerprint: FAEB 9711 A12C F475 812F  18F2 88A9 064D 1835 61EB
#      Subkey fingerprint: F9B7 ABDB BCAC DF95 BE76  CBD0 7DEF 8106 AAFC 390E

* remotes/jnsnow/tags/ide-pull-request:
  block: ignore flush requests when storage is clean
  tests: in IDE and AHCI tests perform DMA write before flushing
  ide: set retry_unit for PIO and FLUSH requests
  ide: refactor retry_unit set and clear into separate function

Signed-off-by: Peter Maydell <[email protected]>