vcpu: add SaveState Vcpu event and response
vcpu: add SaveState Vcpu event and response

Add a new SaveState Vcpu event that signals a paused Vcpu to save its
state and the corresponding Vcpu responses.

Signed-off-by: Ioana Chirca <51e21bdf954d6b11fbd2d2711e2dc884c81ba324@amazon.com>

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