kvm: fix race between machine.Put and machine.Get

m.available.Signal() has to be called under m.mu.RLock, otherwise it can
race with machine.Get:

m.Get			| m.Put
-------------------------------------
m.mu.Lock()		|
Seatching available vcpu|
			| m.available.Signal()
m.available.Wait	|

PiperOrigin-RevId: 259394051

pkg/sentry/platform/kvm/machine.go

@@ -388,7 +388,10 @@ func (m *machine) Get() *vCPU {
 func (m *machine) Put(c *vCPU) {
 	c.unlock()
 	runtime.UnlockOSThread()
+
+	m.mu.RLock()
 	m.available.Signal()
+	m.mu.RUnlock()
 }
 
 // newDirtySet returns a new dirty set.