hypervisor, vmm: Remove inner Mutex protecting VcpuFd

This was added in 7be69edf51 to deal with
changes to the KVM bindings that made run() and set_immediate_exit()
take &mut self. Instead adopt a Box<> value in Vcpu allowing the removal
of this internal Mutex.

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

vmm/src/vm.rs

@@ -3548,7 +3548,7 @@ pub fn test_vm() {
     mem.write_slice(&code, load_addr)
         .expect("Writing code to memory failed");
 
-    let vcpu = vm.create_vcpu(0, None).expect("new Vcpu failed");
+    let mut vcpu = vm.create_vcpu(0, None).expect("new Vcpu failed");
 
     let mut vcpu_sregs = vcpu.get_sregs().expect("get sregs failed");
     vcpu_sregs.cs.base = 0;