KVM: Portability: Move unalias_gfn to arch dependent file

Non-x86 archs don't need this mechanism. Move it to arch, and
keep its interface in common.

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>