kvm tools: Add ability to map guest RAM from hugetlbfs

Add a --hugetlbfs commandline option to give a path to hugetlbfs-map guest
memory (down in kvm__arch_init()).  For x86, guest memory is a normal
ANON mmap() if this option is not provided, otherwise a hugetlbfs mmap.

This maps directly from a hugetlbfs temp file rather than using something
like MADV_HUGEPAGES so that, if the user asks for hugepages, we definitely
are using hugepages.  (This is particularly useful for architectures that
don't yet support KVM without hugepages, so we definitely need to use
them for the whole of guest RAM.)

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>