block: Add bdrv_reopen_queue_free()

Move the code to free a BlockReopenQueue to a separate function.
It will be used in a subsequent patch.

[ kwolf: Also free explicit_options and options, and explicitly
  qobject_ref() the value when it continues to be used. This makes
  future memory leaks less likely. ]

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210708114709.206487-3-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>