Improve performance of HostSimple.addMigratingInVm(Vm)

[Manoel Campos]

FEATURE

Instead of keeping a list of VMs that are migrating into a Host,
which requires searching in such a list every time a VM is requested to be migrated
to that Host (to check if it isn't migrating to that same host already),
we can add a migratingHost attribute to Vm, so that the worst-case time complexity
will change from O(N) to a constant time O(1).