virtio-devices: Implement virtio-device rtc

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>

vm-virtio/src/lib.rs

@@ -33,6 +33,7 @@ pub enum VirtioDeviceType {
     Balloon = 5,
     Fs9P = 9,
     Gpu = 16,
+    Rtc = 17,
     Input = 18,
     Vsock = 19,
     Iommu = 23,
@@ -53,6 +54,7 @@ impl From<u32> for VirtioDeviceType {
             5 => VirtioDeviceType::Balloon,
             9 => VirtioDeviceType::Fs9P,
             16 => VirtioDeviceType::Gpu,
+            17 => VirtioDeviceType::Rtc,
             18 => VirtioDeviceType::Input,
             19 => VirtioDeviceType::Vsock,
             23 => VirtioDeviceType::Iommu,
@@ -76,8 +78,9 @@ impl fmt::Display for VirtioDeviceType {
             VirtioDeviceType::Console => "console",
             VirtioDeviceType::Rng => "rng",
             VirtioDeviceType::Balloon => "balloon",
-            VirtioDeviceType::Gpu => "gpu",
             VirtioDeviceType::Fs9P => "9p",
+            VirtioDeviceType::Gpu => "gpu",
+            VirtioDeviceType::Rtc => "rtc",
             VirtioDeviceType::Input => "input",
             VirtioDeviceType::Vsock => "vsock",
             VirtioDeviceType::Iommu => "iommu",