migration: Remove use of old MigrationParams

We have change in the previous patch to use migration capabilities for
it.  Notice that we continue using the old command line flags from
migrate command from the time being.  Remove the set_params method as
now it is empty.

For savevm, one can't do a:

savevm -b/-i foo

but now one can do:

migrate_set_capability block on
savevm foo

And we can't use block migration. We could disable block capability
unconditionally, but it would not be much better.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

---
- Maintain shared/enabled dependency (Xu suggestion)
- Now we maintain the dependency on the setter functions
- improve error messages

include/migration/migration.h

@@ -39,8 +39,7 @@
 #define QEMU_VM_SECTION_FOOTER       0x7e
 
 struct MigrationParams {
-    bool blk;
-    bool shared;
+    bool unused; /* C doesn't allow empty structs */
 };
 
 /* Messages sent on the return path from destination to source */