Merge remote-tracking branch 'remotes/pmaydell/tags/pull-target-arm-20150721' into staging

target-arm queue:
 * don't sync CNTVCT with kernel all the time (fixes VM time weirdnesses)
 * fix a warning compiling disas/arm-a64 with -Wextra

# gpg: Signature made Tue Jul 21 12:15:33 2015 BST using RSA key ID 14360CDE
# gpg: Good signature from "Peter Maydell <peter.maydell@linaro.org>"

* remotes/pmaydell/tags/pull-target-arm-20150721:
  disas/arm-a64: Add missing compiler attribute GCC_FMT_ATTR
  target-arm: kvm: Differentiate registers based on write-back levels

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>