Merge remote-tracking branch 'remotes/gonglei/tags/bootdevice-next-20141222' into staging

bootdevice: Refactor and improvement

# gpg: Signature made Mon 22 Dec 2014 06:44:08 GMT using RSA key ID DDE30FBB
# gpg: Can't check signature: public key not found

* remotes/gonglei/tags/bootdevice-next-20141222:
  bootdevice: add Error **errp argument for QEMUBootSetHandler
  bootdevice: add validate check for qemu_boot_set()
  bootdevice: add Error **errp argument for qemu_boot_set()
  bootdevice: add Error **errp argument for validate_bootdevices()
  bootdevice: move code about bootorder from vl.c to bootdevice.c

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