KVM: Silence underflow warning in avic_get_physical_id_entry()

Smatch complains that we check cap the upper bound of "index" but don't
check for negatives.  It's a false positive because "index" is never
negative.  But it's also simple enough to make it unsigned which makes
the code easier to audit.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/svm.c

@@ -1272,7 +1272,8 @@ static void init_vmcb(struct vcpu_svm *svm)
 
 }
 
-static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu, int index)
+static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu,
+				       unsigned int index)
 {
 	u64 *avic_physical_id_table;
 	struct kvm_arch *vm_data = &vcpu->kvm->arch;