kvm: tests: Add test to verify MSR_IA32_XSS

Ensure that IA32_XSS appears in KVM_GET_MSR_INDEX_LIST if it can be set
to a non-zero value.

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Change-Id: Ia2d644f69e2d6d8c27d7e0a7a45c2bf9c42bf5ff
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>