KVM: Portability: Split kvm_set_memory_region() to have an arch callout

Moving !user_alloc case to kvm_arch to avoid unnecessary
code logic in non-x86 platform.

Signed-off-by: Zhang Xiantao <xiantao.zhang@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --cc drivers/kvm/kvm.h
Simple merge

diff --cc drivers/kvm/kvm_main.c
Simple merge

diff --cc drivers/kvm/x86.c
Simple merge