block: more read-only changes, related to backing files

Open backing file read-only where possible
Upgrade backing file to read-write during commit, back to read-only after commit
  If upgrade fail, back to read-only. If also fail, "disconnect" the drive.

Signed-off-by: Naphtali Sprei <nsprei@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>