migration: Do not construct JSON description if suppressed

QEMU machine has a property "suppress-vmdesc". When it is enabled, QEMU
will stop attaching JSON VM description at the end of the precopy migration
stream (postcopy is never affected because postcopy never attach that).

However even if it's suppressed by the user, the source QEMU will still
construct the JSON descriptions, which is a complete waste of CPU and
memory resources.

To avoid it, only create the JSON writer object if suppress-vmdesc is not
specified.

Luckily, vmstate_save() already supports vmdesc==NULL, so only a few spots
that are left to be prepared that vmdesc can be NULL now.

When at it, move the init / destroy of the JSON writer object to start /
end of the migration - the JSON writer object is a sub-struct of migration
state, and that looks like the only object that was dynamically allocated /
destroyed within migration process.  Make it the same as the rest objects
that migration uses.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-3-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/migration.h

@@ -552,6 +552,7 @@ void migration_bitmap_sync_precopy(bool last_stage);
 
 /* migration/block-dirty-bitmap.c */
 void dirty_bitmap_mig_init(void);
+bool should_send_vmdesc(void);
 
 /* migration/block-active.c */
 void migration_block_active_setup(bool active);