Merge remote-tracking branch 'quintela/migration.next' into staging

# By Paolo Bonzini (40) and others
# Via Juan Quintela
* quintela/migration.next: (46 commits)
  page_cache: dup memory on insert
  page_cache: fix memory leak
  Fix cache_resize to keep old entry age
  Fix page_cache leak in cache_resize
  migration: inline migrate_fd_close
  migration: eliminate s->migration_file
  migration: move contents of migration_close to migrate_fd_cleanup
  migration: move rate limiting to QEMUFile
  migration: small changes around rate-limiting
  migration: use qemu_ftell to compute bandwidth
  migration: use QEMUFile for writing outgoing migration data
  migration: use QEMUFile for migration channel lifetime
  qemu-file: simplify and export qemu_ftell
  qemu-file: add writable socket QEMUFile
  qemu-file: check exit status when closing a pipe QEMUFile
  qemu-file: fsync a writable stdio QEMUFile
  migration: merge qemu_popen_cmd with qemu_popen
  migration: use qemu_file_rate_limit consistently
  migration: remove useless qemu_file_get_error check
  migration: detect error before sleeping
  ...