KVM: support to init via VSock

[Wenzel]

Support was added in the kvmi crate to init using a VSock socket: https://github.com/Wenzel/kvmi/pull/39

changes required in libmicrovmi:

• the DriverInitParam 's KVMiSocket field should use a SocketType`: https://github.com/Wenzel/kvmi/blob/master/src/lib.rs#L129
• the examples should be updated to take a new parameter to pass the vsock socket: https://github.com/Wenzel/libmicrovmi/blob/master/examples/mem-dump.rs#L19
• kvm driver should be updated