KVM: selftests: Add __vcpu_run() helper

Add __vcpu_run() so that tests that want to avoid asserts on KVM_RUN
failures don't need to open code the ioctl() call.

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