migration: Split migration/channel.c for channel operations

Create an include for its exported functions.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

---
Add proper header

migration/Makefile.objs

@@ -1,5 +1,5 @@
 common-obj-y += migration.o socket.o fd.o exec.o
-common-obj-y += tls.o
+common-obj-y += tls.o channel.o
 common-obj-y += colo-comm.o colo.o colo-failover.o
 common-obj-y += vmstate.o page_cache.o
 common-obj-y += qemu-file.o