selftests: KVM: Replace optarg with arg in guest_modes_cmdline

The parameter arg in guest_modes_cmdline not being used now, and the
optarg should be replaced with arg in guest_modes_cmdline.

And this is the chance to change strtoul() to atoi_non_negative(), since
guest mode ID will never be negative.

Signed-off-by: Shaoqin Huang <shahuang@redhat.com>
Fixes: e42ac77 ("KVM: selftests: Factor out guest mode code")
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Reviewed-by: Vipin Sharma <vipinsh@google.com>
Link: https://lore.kernel.org/r/20230202025716.216323-1-shahuang@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: shaoqinh

tools/testing/selftests/kvm/lib/guest_modes.c

@@ -127,7 +127,7 @@ void guest_modes_cmdline(const char *arg)
 		mode_selected = true;
 	}
 
-	mode = strtoul(optarg, NULL, 10);
+	mode = atoi_non_negative("Guest mode ID", arg);
 	TEST_ASSERT(mode < NUM_VM_MODES, "Guest mode ID %d too big", mode);
 	guest_modes[mode].enabled = true;
 }