block: fully delete bs->file when closing We are reusing bs->file across close/open, which may not cause any known bugs but is a recipe for trouble. Prefer bdrv_delete, and enjoy the new invariant in the implementation of bdrv_delete. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>