qmp/cont: Only activate disks if migration completed

qmp/cont: Only activate disks if migration completed

As the comment says, the activation of disks is for the case where
migration has completed, rather than when QEMU is still during
migration (RUN_STATE_INMIGRATE).

Move the code over to reflect what the comment is describing.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20241206230838.1111496-3-peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>