core: simplify MigrateVmCommand#getAuditLogTypeValue

This method used to contain a ternary-if statement with 3 and 4 levels
which made it less readable. By extracting the those levels to separate
methods, the highest level within that ternary-if statement was reduced
to 2 and now it is more readable.

Change-Id: I54f4e026199187212a872eaef27933a845194b55
Signed-off-by: Arik Hadas <ahadas@redhat.com>