KVM: Move gfn_to_index to x86 specific code

KVM: Move gfn_to_index to x86 specific code

The gfn_to_index function relies on huge page defines which either may
not make sense on systems that don't support huge pages or are defined
in an unconvenient way for other architectures.  Since this is
x86-specific, move the function to arch/x86/include/asm/kvm_host.h.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
(cherry picked from commit 6d9d41e57440e32a3400f37aa05ef7a1a09ced64)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>