core: "LiveMigrateDiskCommand" small code improvements

Small code improvements in a separate PR in order to keep
#305 focused on important changes.

1) Fixed typos variable & method names.
2) Improved variable names.
3) Fixed a potential NPE.
4) Eliminated code warnings
5) Simplified code, improved comments.
6) Handled too long lines or redundant breaking to a newline.
7) Minor code reformatting to improve readability and
create a more consistent code style.

Signed-off-by: Pavel Bar <pbar@redhat.com>