KVM: selftests: Harden and comment XSS / KVM_SET_MSRS interaction

Assert that KVM_SET_MSRS returns '0' or '1' when setting XSS to a
non-zero value.  The ioctl() itself should "succeed", its only the
setting of the XSS MSR that should fail/fault.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>