KVM: Portability: Move memslot aliases to new struct kvm_arch

This patches create kvm_arch to hold arch-specific kvm fileds
and moves fields naliases and aliases to kvm_arch.

Signed-off-by: Zhang Xiantao <xiantao.zhang@intel.com>
Acked-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>