x86: reduce VM size if ACPI to 1GB

Since physical memory is no longer wholly identity mapped,
it is not needed to set the VM size to be larger than
physical memory size. The VM size was 2GB (max physical
memory size of x86 boards) + 1GB (for memory mappings).
So simply shrink the size to 1GB, as the kernel size is
small and we still have a large chunk of space to do
memory mapping.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>