migration: Create migration/blocker.h

This allows us to remove lots of includes of migration/migration.h

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
diff --git a/block/vmdk.c b/block/vmdk.c
index c61b9cc..55581b0 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -31,7 +31,7 @@
 #include "qemu/error-report.h"
 #include "qemu/module.h"
 #include "qemu/bswap.h"
-#include "migration/migration.h"
+#include "migration/blocker.h"
 #include "qemu/cutils.h"
 #include <zlib.h>