core: remove compatibility version from IrsBaseVDSCommandParameters

The IrsBaseVDSCommandParameters class had a compatibility version field
that was really used only by UpgradeStoragePoolVDSCommand, thus it is
moved to UpgradeStoragePoolVDSCommandParameters.

This patch also contains other kinds of minor refactoring:
1. move field declarations to the beginning of the classes
2. remove 'private' prefix from field names

Change-Id: I785e868a1a192af5e5722c5c4562870c69c96b9c
Signed-off-by: Arik Hadas <ahadas@redhat.com>