migration: SCM_RIGHTS for QEMUFile

migration: SCM_RIGHTS for QEMUFile

Define functions to put/get file descriptors to/from a QEMUFile, for qio
channels that support SCM_RIGHTS.  Maintain ordering such that
  put(A), put(fd), put(B)
followed by
  get(A), get(fd), get(B)
always succeeds.  Other get orderings may succeed but are not guaranteed.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-14-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>