skip past an empty region, otherwise we end up generating an infinitely
long e820 map.  (Yes, there *are* real systems that provide e820 maps
with a zero-length region at the end...)

as a result of our memory map editing, so strip out any empty regions.

order?

We now split e820 regions around ourselves, rather than just
truncating the e820 region.  This avoids the worst-case scenario of
losing all memory over 4GB.

It's more important to get the memory map right now that we're
expecting to still be loaded when the OS starts in several situations
(e.g. Linux with UNDI driver, any OS with iSCSI/AoE boot, etc.).