KVM: Portability: Create kvm_arch_vcpu_runnable() function

This abstracts the detail of x86 hlt and INIT modes into a function.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Acked-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>