kernel: KVM: x86/xen: Initialize Xen timer only once

In the Linux kernel, the following vulnerability has been resolved: KVM: x86/xen: Initialize Xen timer only once Add a check for existing xen timers before initializing a new one. Currently kvmxeninittimer is called on every KVMXENVCPUATTRTYPETIMER, which is causing the following ODEBUG crash whe...