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>

cloud-hypervisor/src/main.rs

@@ -34,8 +34,8 @@ use vmm::vm_config::IvshmemConfig;
 use vmm::vm_config::{
     BalloonConfig, ConsoleConfig, DeviceConfig, DiskConfig, FsConfig, GenericVhostUserConfig,
     LandlockConfig, NetConfig, NumaConfig, PciSegmentConfig, PlatformConfig, PmemConfig,
-    RateLimiterGroupConfig, RngConfig, SerialConfig, TpmConfig, UserDeviceConfig, VdpaConfig,
-    VmConfig, VsockConfig,
+    RateLimiterGroupConfig, RngConfig, RtcConfig, SerialConfig, TpmConfig, UserDeviceConfig,
+    VdpaConfig, VmConfig, VsockConfig,
 };
 use vmm_sys_util::eventfd::EventFd;
 use vmm_sys_util::signal::block_signal;
@@ -404,6 +404,12 @@ fn get_cli_options_sorted(
             .help(RngConfig::SYNTAX)
             .default_value(default_rng)
             .group("vm-config"),
+        Arg::new("rtc")
+            .long("rtc")
+            .help(RtcConfig::SYNTAX)
+            .num_args(0..=1)
+            .default_missing_value("")
+            .group("vm-config"),
         Arg::new("seccomp")
             .long("seccomp")
             .num_args(1)
@@ -1043,6 +1049,7 @@ mod unit_tests {
             iommu: false,
             numa: None,
             watchdog: false,
+            rtc: None,
             #[cfg(feature = "guest_debug")]
             gdb: false,
             pci_segments: None,