KVM: selftests: Capture per-vCPU GPA in perf_test_vcpu_args

Capture the per-vCPU GPA in perf_test_vcpu_args so that tests can get
the GPA without having to calculate the GPA on their own.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20211111000310.1435032-7-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>