core: rename MigrateVmCommand#getVdsDestination

MigrateVmCommand#getVdsDestination used to return the name of the
destination VDS for audit log messages. Its name was confusing as we
already have method called getDestinationVds in MigrateVmCommand and it
didn't reflect the purpose of this method well. Thus it is renamed to
getDestinationVdsName.

Change-Id: Iab5e534d298bd99bb6b2692d58c378effae3760f
Signed-off-by: Arik Hadas <ahadas@redhat.com>