At the moment, Xen has a 16TB limit on x86, so this has no practical
impact, aside for silencing compiler truncation warnings.
Also, the rest of the GUI/u2mfn code and protocol would need to be
expanded to use 64-bit mfns for Qubes to actually work there.
This is preparation for pvgrub support, where all VM kernel files will
be installed inside of VM instead of dom0.
But also the same could be used to prepare VM kernel image from any dom0
kernel.
