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>

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -667,6 +667,8 @@ components:
         watchdog:
           type: boolean
           default: false
+        rtc:
+          $ref: "#/components/schemas/RtcConfig"
         pvpanic:
           type: boolean
           default: false
@@ -1087,6 +1089,21 @@ components:
         src:
           type: string
 
+    RtcConfig:
+      type: object
+      properties:
+        id:
+          type: string
+        pci_segment:
+          type: integer
+          format: int16
+        pci_device_id:
+          type: integer
+          format: uint8
+        iommu:
+          type: boolean
+          default: false
+
     BalloonConfig:
       required:
         - size