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This change will allow us to get accurate time over ptp in guests started from a MSHV-virtualized Linux host. Implementing it as a virtio device is preferable to using the existing kvm_ptp because: kvm_ptp relies on hypercalls that only exist on host kernels running kvm. Virtio-rtc gives us more flexibility in what clock types we want to provide. We can later extend the device to implement multiple clocks (smeared UTC, TAI, monotonic, etc.). Virtio-rtc protocol supports alarms. Alarms may later enable usecases where the guests can do their own VM lifecycle management without relying on a host-side orchestrator. Implement device backend for virtio-rtc. Currently this implementation encompasses: 1. CONFIG, CAP, READ, CROSSCAP (returns false) 2. One PTP clock is presented of type VIRTIO_RTC_CLOCK_UTC_MAYBE_SMEARED with leap_second_smearing VIRTIO_RTC_SMEAR_UNSPECIFIED The device is disabled by default, requiring --rtc to be passed Not implemented but theoretically supported by virtio-rtc is: 1. Cross-timestamping support 2. The alarm queue Fixes #7730 Signed-off-by: Cameron Baird <cameronbaird@microsoft.com>