acpi: Fix early-ACPI partial linear map

When running on a KVM host without EPT, OSv used to crash if ran 1GB
or less memory. The crash was in early acpi initialization, which called
AcpiOsMapMemory, which called linear_map() on the small chunk of physical
memory requested by ACPI.

It turns out that the chunk requested was just 4096 bytes, which caused
our linear_map() implementation to break down a huge-page into small-pages.
This didn't work - I'm don't know yet what exactly is the bug or why it
only shows up when the host doesn't use EPT, or if the memory is smaller
than 1GB...

But in any case, since we know our complete linear map always uses huge
pages, I think it makes no sense to map a single small page, and we can
just map a whole huge page. This will cause linear_map() not to try to
break up any huge pages, and this bug goes away.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>