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

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor