hvmloader: add high memory e820 region if needed

hvmloader: add high memory e820 region if needed

If the MMIO hole is large and hvmloader needs to relocate memory to
immediately above the 4 GiB boundary, the e820 presented to the guest
will not have a RAM region above 4 GiB.

e.g., a guest with 3 GiB of memory and a 2 GiB MMIO hole will only see
2 GiB.

The required e820 memory region above 4 GiB needs to be added, and not
just filled in.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>