KVM: selftests: Use shorthand local var to access struct perf_tests_args

Use 'pta' as a local pointer to the global perf_tests_args in order to
shorten line lengths and make the code borderline readable.

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: <20211111000310.1435032-6-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>