Split qemu_savevm_state_begin to:
  qemu_savevm_state_header   That writes the initial file header.
  qemu_savevm_state_begin    That sets up devices and does the first
                             device pass.

Used later in postcopy.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Reviewed-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>