Merge remote-tracking branch 'remotes/juanquintela/tags/migration/20170206' into staging

migration/next for 20170206

# gpg: Signature made Mon 06 Feb 2017 16:13:26 GMT
# gpg:                using RSA key 0xF487EF185872D723
# gpg: Good signature from "Juan Quintela <quintela@redhat.com>"
# gpg:                 aka "Juan Quintela <quintela@trasno.org>"
# Primary key fingerprint: 1899 FF8E DEBF 58CC EE03  4B82 F487 EF18 5872 D723

* remotes/juanquintela/tags/migration/20170206:
  postcopy: Recover block devices on early failure
  Postcopy: Reset state to avoid cleanup assert
  vmstate registration: check return values
  migration: Check for ID length
  vmstate_register_with_alias_id: Take an Error **
  migration: create Migration Incoming State at init time

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