Allow the vnc server to run when using crosvm as vmm

The vnc server sends input through unix sockets to crosvm's virtio
input devices and receives screen updates through a vsock socket.
Performance is not the best, but it's usable.

Bug: 128852363
Bug: 127361363
Test: run locally
Change-Id: I2b2be2c3f338e7624e390879293ce61186dded38