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

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

# By Peter Lieven (9) and others
# Via Juan Quintela
* quintela/migration.next: (22 commits)
  Use qemu_put_buffer_async for guest memory pages
  Add qemu_put_buffer_async
  Use writev ops if available
  Store the data to send also in iovec
  Update bytes_xfer in qemu_put_byte
  Add socket_writev_buffer function
  Add QemuFileWritevBuffer QemuFileOps
  migration: use XBZRLE only after bulk stage
  migration: do not search dirty pages in bulk stage
  migration: do not sent zero pages in bulk stage
  migration: add an indicator for bulk state of ram migration
  migration: search for zero instead of dup pages
  bitops: unroll while loop in find_next_bit()
  buffer_is_zero: use vector optimizations if possible
  cutils: add a function to find non-zero content in a buffer
  move vector definitions to qemu-common.h
  savevm: Fix bugs in the VMSTATE_VBUFFER_MULTIPLY definition
  savevm: Add VMSTATE_STRUCT_VARRAY_POINTER_UINT32
  savevm: Add VMSTATE_FLOAT64 helpers
  savevm: Add VMSTATE_UINTTL_EQUAL helper
  ...