openvmm tdx: use timer virtualization for lower VTLs

Implements https://github.com/microsoft/openvmm/issues/2028

This PR implements hardware timer virtualization for lower VTLs on TDX CVM usingL2-VM TSC deadline timer, an architectural capability provided by TDX module. This improves the CVM performance by eliminating guest exits to hypervisor for timer arming and expiry notifications for lower VTL's timer requirements.

The related changes in OHCL-kernel is implemented by https://github.com/microsoft/OHCL-Linux-Kernel/pull/107

Background - TDX L2-VM TSC Deadline Timer

This allows VTL2 to set an execution deadline for lower VTLs. If the lower VTL is running when the deadline time arrives, it exits to VTL2 with exit reason VmxExitBasic::TIMER_EXPIRED. If the TSC deadline is in the past during entry into lower VTL (i.e., TSC deadline value is lower than the current virtual TSC value), it will immediately exit back to VTL2 with exit reason VmxExitBasic::TIMER_EXPIRED.

The TSC deadline is set using TDG.VP.WR for TDVPS.TSC_DEADLINE[L2-VM Index].

Implementation

With these changes, openvmm evaluates earliest deadline across all lower VTLs and sets it in a tdx_vp_context ->tdx_l2_tsc_deadline_state that is shared with mshv_vtl driver.
During entry into lower VTL, mshv_vtl driver makes the TDG.VP.WR call to set the deadline when an update is needed.

Changes

Added HardwareIsolatedGuestTimer trait as an abstraction for managing lower VTL timer deadlines.
Moved current VmTime interface as default/fallback implementation into this trait.
Added TdxTscDeadlineService to implement the TDX specific timer virtualization.