core: [cleanup] extract building parameters code from execute methods

Extract the sections that build parameters for internal commands from
the execute methods of CreateAllSnapshotsFromVmCommand,
RemoveAllVmImagesCommand and RemoveDiskCommand. that way, the execute
methods are shorter and more readable.

Change-Id: Iedcdcefe63c75927ed709faa9c1a64c5e983bc12
Signed-off-by: Arik Hadas <ahadas@redhat.com>