hypervisor: add a generic guest-clock save/restore abstraction

Preserving the guest clock across pause/resume and snapshot/restore is
currently open-coded in the VMM against the x86-only
get_clock/set_clock. aarch64 needs the same correction but via a
different mechanism (i.e. the architected counter, CNTVCT). Having a
common backend-agnostic interface that VMM can drive uniformly allows us
to keep the architecture details behind the Hypervisor abstraction.

This commit only introduces the abstraction while the future commits
will actually move the implementation to use it.

Use this opportunity to fix the full path to get SystemTime as well.

Suggested-by: Sebastien Boeuf <sboeuf@meta.com>
Signed-off-by: Atish Patra <atishp@meta.com>

hypervisor/src/mshv/mod.rs

@@ -78,7 +78,7 @@ pub use {
 };
 
 #[cfg(target_arch = "x86_64")]
-use crate::ClockData;
+use crate::{ClockData, ClockState};
 #[cfg(target_arch = "aarch64")]
 use crate::arch::aarch64::gic::{Vgic, VgicConfig};
 #[cfg(target_arch = "aarch64")]
@@ -2200,6 +2200,18 @@ impl vm::Vm for MshvVm {
             .map_err(|e| vm::HypervisorVmError::SetClock(e.into()))
     }
 
+    /// Capture the partition reference time for snapshot/migration.
+    #[cfg(target_arch = "x86_64")]
+    fn snapshot_clock(&self) -> vm::Result<Option<ClockState>> {
+        Ok(Some(self.get_clock()?.with_realtime_filled()))
+    }
+
+    /// Restore the partition reference time before the vCPUs resume.
+    #[cfg(target_arch = "x86_64")]
+    fn restore_clock(&self, state: &ClockState) -> vm::Result<()> {
+        self.set_clock(state)
+    }
+
     /// Downcast to the underlying MshvVm type
     fn as_any(&self) -> &dyn Any {
         self