hypervisor, vmm: Limit max number of vCPUs to hypervisor maximum

On KVM this is provided by an ioctl, on MSHV this is constant. Although
there is a HV_MAXIMUM_PROCESSORS constant the MSHV ioctl API is limited
to u8.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

hypervisor/src/hypervisor.rs

@@ -133,4 +133,7 @@ pub trait Hypervisor: Send + Sync {
     fn get_guest_debug_hw_bps(&self) -> usize {
         unimplemented!()
     }
+
+    /// Get maximum number of vCPUs
+    fn get_max_vcpus(&self) -> u32;
 }