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
diff --git a/migration/exec.c b/migration/exec.c
index aba9089..57a9335 100644
--- a/migration/exec.c
+++ b/migration/exec.c
@@ -20,6 +20,7 @@
 #include "qemu/osdep.h"
 #include "qapi/error.h"
 #include "qemu-common.h"
+#include "channel.h"
 #include "migration/migration.h"
 #include "io/channel-command.h"
 #include "trace.h"