block: use drained section in bdrv_close bdrv_close is used when ejecting a medium. Use a drained section to ensure that all I/O goes to either the old medium or the bitbucket. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 1450867706-19860-2-git-send-email-pbonzini@redhat.com Signed-off-by: Max Reitz <mreitz@redhat.com>