Merge remote-tracking branch 'remotes/juanquintela/tags/migration/20170316' into staging

migration/next for 20170316

# gpg: Signature made Thu 16 Mar 2017 08:21:51 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/20170316:
  postcopy: Check for shared memory
  RAMBlocks: qemu_ram_is_shared
  vmstate: fix failed iotests case 68 and 91
  migration/block: Avoid invoking blk_drain too frequently
  migration: use "" as the default for tls-creds/hostname
  Change the method to calculate dirty-pages-rate

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