core: cleanup in MigrateVmCommand

Extract the code in MigrateVmCommand#perform method which is related
only to the parameters initialization to separate methods, to make it
shorter and easier to read.

This patch also includes other minor refactoring in this class.

Change-Id: I138ac8c64e7c0fe70c579e80313b56d4d1d47848
Signed-off-by: Arik Hadas <ahadas@redhat.com>