Validate backup size during restore

Reasons:
 - A user is able to pass a backup that is larger
   than the volume/disk of the instance can handle.

Changes:
 - Add validation to backup size according to volume
   or flavor size of the disk;
 - Add tests.

Change-Id: Id549cff2e572a0ada632759bd9fcbbb67edf6ae0
Closes-Bug: #1304603
Merge "Validate backup size during restore"