Merge remote-tracking branch 'remotes/dgilbert/tags/pull-migration-20180425a' into staging

Migration pull for 2.13

Alexey Perevalov postcopy blocktime statistics
Xiao Guangrong's compression performance improvements

# gpg: Signature made Wed 25 Apr 2018 20:21:13 BST
# gpg:                using RSA key 0516331EBC5BFDE7
# gpg: Good signature from "Dr. David Alan Gilbert (RH2) <dgilbert@redhat.com>"
# Primary key fingerprint: 45F5 C71B 4A0C B7FB 977A  9FA9 0516 331E BC5B FDE7

* remotes/dgilbert/tags/pull-migration-20180425a:
  migration: remove ram_save_compressed_page()
  migration: introduce save_normal_page()
  migration: move calling save_zero_page to the common place
  migration: move calling control_save_page to the common place
  migration: move some code to ram_save_host_page
  migration: introduce control_save_page()
  migration: detect compression and decompression errors
  migration: stop decompression to allocate and free memory frequently
  migration: stop compression to allocate and free memory frequently
  migration: stop compressing page in migration thread
  migration: add postcopy total blocktime into query-migrate
  migration: add blocktime calculation into migration-test
  migration: postcopy_blocktime documentation
  migration: calculate vCPU blocktime on dst side
  migration: add postcopy blocktime ctx into MigrationIncomingState
  migration: introduce postcopy-blocktime capability

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