KVM: selftests: Use vcpu_access_device_attr() in arm64 code

Use vcpu_access_device_attr() in arm's arch_timer test instead of
manually retrieving the vCPU's fd.  This will allow dropping vcpu_get_fd()
in a future patch.

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