Reduce the use of size_t.

Many uses have more specific types that should be used instead. In
particular, size_t was used for register sized types so uintreg_t is
introduced to be more explicit and portable.

Change-Id: Ib8038bdd9cd164e6d9b1f42d10d4b1654a665997
diff --git a/src/vm.c b/src/vm.c
index c840c2a..02b60ff 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -69,7 +69,7 @@
 }
 
 /* TODO: Shall we use index or id here? */
-void vm_start_vcpu(struct vm *vm, size_t index, ipaddr_t entry, size_t arg)
+void vm_start_vcpu(struct vm *vm, size_t index, ipaddr_t entry, uintreg_t arg)
 {
 	struct vcpu *vcpu = &vm->vcpus[index];
 	if (index < vm->vcpu_count) {