Merge remote-tracking branch 'remotes/juanquintela/tags/migration/20150316' into staging

migration/next for 20150316

# gpg: Signature made Mon Mar 16 13:36:37 2015 GMT using RSA key ID 5872D723
# gpg: Can't check signature: public key not found

* remotes/juanquintela/tags/migration/20150316:
  pc: Disable vmdesc submission for old machines
  migration: Allow to suppress vmdesc submission
  migration: Read JSON VM description on incoming migration
  rename save_block_hdr to save_page_header
  save_block_hdr: we can recalculate the cont parameter here
  save_xbzrle_page: change calling convention
  ram_save_page: change calling covention
  ram_find_and_save_block:  change calling convention
  ram: make all save_page functions take a uint64_t parameter
  Add migrate_incoming
  Add -incoming defer

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