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>

arch/src/aarch64/mod.rs

@@ -66,7 +66,7 @@ pub struct EntryPoint {
 
 /// Configure the specified VCPU, and return its MPIDR.
 pub fn configure_vcpu(
-    vcpu: &Arc<dyn hypervisor::Vcpu>,
+    vcpu: &dyn hypervisor::Vcpu,
     id: u32,
     boot_setup: Option<(EntryPoint, &GuestMemoryAtomic<GuestMemoryMmap>)>,
 ) -> super::Result<u64> {