KVM: selftests: Convert xss_msr_test away from VCPU_ID

Convert xss_msr_test to use vm_create_with_one_vcpu() and pass around a
'struct kvm_vcpu' object instead of using a global VCPU_ID.  Note, this
is a "functional" change in the sense that the test now creates a vCPU
with vcpu_id==0 instead of vcpu_id==1.  The non-zero VCPU_ID was 100%
arbitrary and added little to no validation coverage.  If testing
non-zero vCPU IDs is desirable for generic tests, that can be done in the
future by tweaking the VM creation helpers.

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