webadmin: Correct 64-bit VM memory limit validation

The UnitVmModel.UpdateMaximalVmMemSize method wasn't called on
Cluster_SelectedItemChanged, so the validation used the initial value of
262144 MB which is obsolete.

Also changed the default value for ConfigValues.VM64BitMaxMemorySizeInMB from
262144 to more up-to-date 524288 (supported since 3.0).

Change-Id: I16562b554a133e9c753a859d6f3f5a348eaa72cc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=954021
Signed-off-by: Martin Betak <mbetak@redhat.com>