remove VcpuEvent::Exit
remove VcpuEvent::Exit

When the emulation thread issues a VcpuEvent::Exit to a certain vCPU,
the vCPU state machine finishes execution but also sends a signal to the
emulation thread to close the VMM. This seems redundant. The emulation thread
can simply close the VMM directly.

Signed-off-by: Serban Iorga <4cd75ae1c2f5ea126376f5daa1ce9f5f3016cb24@amazon.com>

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