bdrv_close_all() asserts that no jobs are running any more, so we need
to cancel all jobs first to avoid failing the assertion.

Fixes: b55a3c88
Reported-by: Nini Gu <ngu@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210309121814.31078-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>