vmm: retrieve timebase-frequency from KVM instead of hardcoding

The RISC-V device tree's timebase-frequency was hardcoded to 10 MHz
(0x989680). Actual hardware uses different frequencies.

Read the timebase frequency from KVM_GET_ONE_REG via
KVM_REG_RISCV_TIMER (offset 0, kvm_riscv_timer.frequency),
thread it through the VMM to arch to FDT layers, and fall back to
the 10 MHz default when KVM returns no value.

Signed-off-by: Meng Zhuo <mengzhuo@iscas.ac.cn>

vmm/src/cpu.rs

@@ -642,6 +642,11 @@ impl Vcpu {
         self.vcpu.run()
     }
 
+    #[cfg(target_arch = "riscv64")]
+    pub fn get_timebase_frequency(&self) -> result::Result<u64, hypervisor::HypervisorCpuError> {
+        self.vcpu.get_timebase_frequency()
+    }
+
     #[cfg(feature = "sev_snp")]
     pub fn set_sev_control_register(&self, vmsa_pfn: u64) -> Result<()> {
         self.vcpu