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/vm_config.rs

@@ -451,6 +451,12 @@ impl Default for RngConfig {
     }
 }
 
+#[derive(Clone, Debug, Default, PartialEq, Eq, Deserialize, Serialize)]
+pub struct RtcConfig {
+    #[serde(flatten)]
+    pub pci_common: PciDeviceCommonConfig,
+}
+
 impl ApplyLandlock for RngConfig {
     fn apply_landlock(&self, landlock: &mut Landlock) -> LandlockResult<()> {
         // Rng Path only need read access
@@ -1051,6 +1057,8 @@ pub struct VmConfig {
     pub numa: Option<Box<[NumaConfig]>>,
     #[serde(default)]
     pub watchdog: bool,
+    #[serde(default)]
+    pub rtc: Option<RtcConfig>,
     #[cfg(feature = "guest_debug")]
     #[serde(default)]
     pub gdb: bool,