vmm: only use KVM_ARM_VCPU_PMU_V3 if available

Having PMU in guests isn't critical, and not all hardware supports
it (e.g. Apple Silicon).

CpuManager::init_pmu already has a fallback for if PMU is not
supported by the VCPU, but we weren't getting that far, because we
would always try to initialise the VCPU with KVM_ARM_VCPU_PMU_V3, and
then bail when it returned with EINVAL.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

hypervisor/src/kvm/mod.rs

@@ -330,7 +330,7 @@ impl KvmVm {
         Ok(VfioDeviceFd::new_from_kvm(device_fd))
     }
     /// Checks if a particular `Cap` is available.
-    fn check_extension(&self, c: Cap) -> bool {
+    pub fn check_extension(&self, c: Cap) -> bool {
         self.fd.check_extension(c)
     }
 }