blockdev: unset inappropriate flags when changing medium

Most importantly, this removes BDRV_O_TEMPORARY, to avoid unlink()ing an
image which replaces a snapshotted one.

Signed-off-by: Alyssa Milburn <fuzzie@fuzzie.org>
Message-id: 20160206133618.GA16635@li141-249.members.linode.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
