webadmin: fix 64 bit VM memory limit validation

The supported maximum for 64 bit VMs is 2TB

Change-Id: I2a32b1e16adff67a5c4656106e10107d840a61fb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=954021
Signed-off-by: Michal Skrivanek <michal.skrivanek@redhat.com>