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>

virtio-devices/src/lib.rs

@@ -27,6 +27,7 @@ pub mod mem;
 pub mod net;
 mod pmem;
 mod rng;
+mod rtc;
 pub mod seccomp_filters;
 mod thread_helper;
 pub mod transport;
@@ -54,6 +55,7 @@ pub use self::mem::{BlocksState, Mem, VIRTIO_MEM_ALIGN_SIZE, VirtioMemMappingSou
 pub use self::net::{Net, NetCtrlEpollHandler};
 pub use self::pmem::Pmem;
 pub use self::rng::Rng;
+pub use self::rtc::Rtc;
 pub use self::vdpa::{Vdpa, VdpaDmaMapping};
 pub use self::vsock::Vsock;
 pub use self::watchdog::Watchdog;