selftests: KVM: Replace optarg with arg in guest_modes_cmdline

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: e42ac777d661 ("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>