Change virtnbdbackup bitmap namees to avoid overwriting

[G1org1owo]

Problem

When a full backup is created, old bitmaps are deleted/overwritten. For this reason, there's a specific edge case when a user follows these steps:

• creates a full backup with --frequency x
• creates an incremental backup with --frequency x
• creates a full backup with --frequency y
• creates an incremental backup with --frequency x

and other similar situations.

Proposed solution

Changing the naming convention used by virtnbdbackup might resolve this issue. This needs to be investigated and there might be a need for a PR to virtnbdbackup