x86: Prefer multiboot-provided e820 over bios-provided e801 memory info.

x86: Prefer multiboot-provided e820 over bios-provided e801 memory info.

Some UEFI systems do not provide e820 information. In this case we
should take the detailed memory map provided by a multiboot-capable
loader, rather than rely on very conservative values from the e801
bios call. Using the latter on any modern system really hardly makes
good sense.

[Excellent candidate for 4.1 backport]

Signed-off-by: Keir Fraser <keir@xen.org>
Tested-by: Jonathan Tripathy <jonnyt@abpni.co.uk>