migration: move only_migratable to MigrationState

One less global variable, and it does only matter with migration.

We keep the old "--only-migratable" option, but also now we support:

  -global migration.only-migratable=true

Currently still keep the old interface.

Hmm, now vl.c has no way to access migrate_get_current(). Export a
function for it to setup only_migratable.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1498536619-14548-7-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/misc.h

@@ -55,4 +55,6 @@ bool migration_has_finished(MigrationState *);
 bool migration_has_failed(MigrationState *);
 /* ...and after the device transmission */
 bool migration_in_postcopy_after_devices(MigrationState *);
+void migration_only_migratable_set(void);
+
 #endif

include/sysemu/sysemu.h

@@ -15,7 +15,6 @@
 /* vl.c */
 
 extern const char *bios_name;
-extern int only_migratable;
 extern const char *qemu_name;
 extern QemuUUID qemu_uuid;
 extern bool qemu_uuid_set;