core: Add validation for Vm memory hard limits

The vdc options VM64BitMaxMemorySizeInMB and VM64BitMaxMemorySizeInMB
were not checked at all, only the os-info were used to issue a warning.

Now we check those against the appropriate cluster version at the VM
run.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1104774
Change-Id: I46c22acc77867abc79cd54ef6113ab2da005fd94
Signed-off-by: Martin Betak <mbetak@redhat.com>